Happines is a question of chemistry, quite litterally!

First of all, we need to clarify a fundamental aspect of the issue, which is to pose the meaning of a highly subjective feeling as an axiom. This is certainly not an easy thing to do, but we have to start somewhere. It is well known that our personal happiness is a state of mind that is triggered as a consequence of actions or events that involve us directly or other living beings around us. Happiness can be described as a positive emotional state, a feeling of well-being and satisfaction, and this feeling can arise through two philosophically opposite paths, material attainment and empathic satisfaction. But analyzing each and every facet of this already highly debated feeling in history is of little use, so let us look at how and where it develops at the chemical level and what makes being happy possible. 
From a chemical point of view, 4 neurotransmitters related to states of happiness are known: 

- Dopamine: reward hormone, it is activated by eating and undertaking self-directed work or actions, it is the reward that the brain gives us to induce and entice us to perform a positive activity for our survival; once the action is completed, its production stops, and we are induced to look for another positive action to undertake (the anticipation of pleasure...no?). The same is true when a predator sees prey, entering a state of arousal conducive to the hunting effort.  

- Oxytocin: love hormone: induces a feeling of trust, of primary importance in social mammals. It is produced through physical contact, and is one reason why parents lick puppies or we feel good when we are cuddled. Neuronal pathways established during brain development through oxytocin allow individuals to develop a more or less intense bond with the future "pack" once they become adults. It is also related to feelings of loneliness and nostalgia. It has other tasks, i.e., it intervenes during childbirth and lactation and is responsible for promoting smooth muscle contraction, facilitating fetal and milk outflow (it does not control milk production). Among its psychoactive activities it regulates social, sexual, parenting and "couple" behaviors as it promotes the establishment of the paternal instinct and parental care.
These two work in antithesis, dopamine driving a mammal toward a reward (e.g., food, typically depleted quickly in the herd's territory), the latter causing the animal to return toward the herd once the reward is obtained.

- Serotonin: plays functions in mood control, appetite, sleep, wound healing (through platelets) and regulation of sexual desire, has an important role in the incidence of depressive events. It is involved in the pursuit of higher social status (the top dog or matriarch), so our brains are built to reward us with serotonin whenever we feel we are in a dominant position in a group (whereas in case we are in an inferior position it is cortisol that is released, the stress hormone). However, this condition is cyclical, as serotonin is "digested" very quickly, and it is then necessary to replay the dominant behaviors in order to secrete new ones (superiority on a moral level is also rewarded). It is characteristically produced from an essential amino acid, tryptophan (also found in chocolate) and therefore cannot be produced directly by the body but must be acquired through nutrition. In the brain it serves to transmit messages between nerve cells, but most of it is found in the gut where it regulates digestive flow and intestinal secretions. An excess of serotonin leads to constipation while a defect leads to diarrhea. It also serves to regulate the rate of digestion and the 'expulsion of unhealthy foods through nausea. It is evident now why there is a strong correlation between psychological and physical stress. Serotonin is also a biological precursor of melatonin, which controls sleep quality and duration and promotes relaxation and falling asleep.

- Endorphin: induces a euphoria that has analgesic and relaxing effects. Its main purpose is to mask pain during a fight or hunt. It is released in small doses during movement and while we laugh, and is therefore linked to activities such as exercise and reproduction, taking part in appetite control and thermoregulation. Endorphins are also released during massage and acupuncture.  
These neurotransmitters are not stable but are metabolized very quickly. One of their functions is to mask the constant presence of the alarm hormone cortisol, and when these four happiness hormones are lacking, the body is left at the mercy of the latter. Cortisol promotes survival through actions to avoid pain. The neural connections that are made as cortisol is released, such as when we touch a hot surface, allow us to experience feelings of revulsion more quickly when these negative situations occur to us again (protecting us from getting burned a second time). The action of cortisol is so intense that it causes us to commit whatever actions are necessary to avoid the dangerous situation (such as pouring lands a pot full of pasta, giving up eating). Hunger itself also produces cortisol.

Any situation that activates a cortisol-related neuronal circuit creates a feeling of discomfort in us, and thus induces us to seek behaviors that can suppress this discomfort, even though they may not be positive behaviors if protracted over the long term. Cortisol is not only induced by the presence of fire or predators, but also by the disappointment of the expectation of a reward (i.e., when we miss a catch, or fail an exam, or lose an important client). Social disappointments also induce a release of cortisol, and when we feel that our social status is in jeopardy malaise arises.  The human ability to anticipate danger allows us to avoid getting hurt, but at the same time fills us with cortisol. Cortisol is metabolized in about two hours, an extremely long time, and the greater the amount of cortisol in our system, the greater the likelihood of detecting potential dangers and secreting further cortisol, setting us up in a cycle of malaise. For us humans, this cycle finds an end as soon as our brains are distracted by positive stimuli, and two hours of continuous distraction allows us to dispose of the cortisol released by a danger event (which is why we enjoy movies and television so much).

These signaling systems evolved to reward a mammal, in the specific case ourselves, whenever we adopt a positive attitude for our survival by increasing our reproductive success, and not to be produced continuously in massive quantities (as is commonly tended to be believed). Also common to all mammals are the centers for controlling their release: Hypothalamus, Pituitary Gland, Hippocampus and Amygdala (the limbic system in short). A practical example of dopamine production is the case of a mammal (us) seeing an easy meal: the brain immediately releases dopamine, and each step that brings us closer to the "object of desire" stimulates a continuous release of more and more dopamine until we reach it, at which time the dopamine release stops since its purpose is to induce us to climb a tree to get an apple, and not make us feel good about getting it. The process starts again as soon as a second meal is identified. The neuronal circuits that are consolidated at the moment of dopamine release, i.e., during meal-seeking in the aforementioned case, are those "memories" that prompt the animal to repeat the action it completed in the past by obtaining as a reward the release of the neurotransmitter, as well as a juicy apple. Typically, these release pathways are consolidated during infancy (and are highly specific and "personalized" in that they are built on our experiences), a time when parental care in mammals consolidates certain behaviors in pups (and paws consolidate negative ones); at a later age these neural pathways can still be formed, only they require more assiduous repetition for consolidation. To always keep in mind is that this intricate reward system has evolved by filtering through the mechanism of natural selection, i.e., a process designed not to make us feel good and gratify ourselves but to make us survive, whereupon the reward system in a non-natural environment can also lead to the onset of deleterious behaviors in that, for example, excessive nutrition leads to a deterioration of our physical health but at the same time to a massive production of pleasure hormones, which linked with the extreme availability of highly caloric food leads, and will lead, to the onset of serious obesity epidemics. The mammalian brain develops thought in two extremely simple opposite modes, an action apt to receive a reward (and thus the release of one or more of the neurotransmitters mentioned above) or an action apt to avoid punishment (thus avoiding being alone with our obnoxious cortisol).

It is therefore a very simple reward and punishment system that regulates the distinction between actions that can lead to survival and death. The modification of neural pathways is extremely complicated because an already established neural pathway is not easily disassembled, and being very well established it has a very smooth and fast "current" flow, but strategies can be implemented to bypass a deleterious or fallacious neural pathway through new experiences (and thus the construction of new neural pathways). So how do we go about replacing a habit that we feel is positive (and indeed on a chemical level it is) but that is challenging us in everyday life? 

We will find out in the next installment of this series.