Serotonin : Blood-based ("serum") chemical that constricts blood-vessels ("tonus")
Serotonin is important for mood regulation. Low levels of serotonin have been associated with depression, irritability, and other badnesses. Many anti-depressant drugs work by increasing serotonin levels in the brain, and many recreational drugs also work by increasing brain serotonin levels (which often leads to the post-hgh comedown once the serotonin has been used up). The rationale therefore is that if you can keep your brain serotonin at a healthy level, you're less likely to suffer from depression.
In a paper from 2007, Simon Young summarises ways in which we can increase our serotonin levels without drugs. In brief, these are:
(References are in the Young 2007 paper below)
1) Be happy! Ok this is slightly circular, but the point is that doing things that you enjoy, such as being social, may be a pretty direct way to boost serotonin. In addition, Young suggests that positive moods (and the related increase in serotonin production) can be induced, either through therapy or self-induction. As Young puts it, the relationship between mood and serotonin production may be 2-way.
2) Look at Bright Lights. Light is a powerful controller of our natural daily hormone cycles, and it seems that it also has a big influence on serotonin production. This may explain why many suffer from Seasonal Affective Disorder (SAD) in the low-light winter months (less light = less serotonin). Therefore, try not to spend the morning daylight hours under a blanket in a dark room looking at your phone (like I do).
3) Exercise. There is some debate over whether exercise really induces the production of more serotonin in the brain, but even if it doesn't, exercise has been shown to improve mood (see point 1) in healthy people and in sufferers of mild depression. And your heart and waistline will thank you! Apparently exercise-to-exhaustion causes the bigger increase in serotonin (which would explain those self-righteous happy joggers).
4) Diet. Your body needs an amino acid called trytophan to make serotonin. There is some evidence that eating purified tryptophan may therefore increase serotonin levels in the brain, but it's not clear whether you can get the same benefit from naturally-occurring sources of tryptophan. Tryptophan is not a very abundant amino acid, and has to compete with other amino acids for access to the brain. Therefore if you eat a high-protein diet (proteins are just chains of amino acids stuck together) you may actually be reducing the proportion of tryptophan in your blood. It's possible that eating foods with a high tryptophan/protein ratio might help, but I think it's fair to say the evidence is still inconclusive on this one. In any case, it probably doesn't hurt to make sure you are eating enough tryptophan as this is the only way your body can get it (hence why it is an "essential" amino acid). Here is a handy table from Wikipedia showing the foods with the highest tryptophan/protein ratio (Milk, Sesame seeds, sunflower seeds, soybeans, spirulina & cheese come out on top).
I'm off to eat cheese and stare at the sun.
Take care of your brain!
Young 2007 paper: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2077351/#__ffn_sectitle
Finally: All you ever need to know about serotonin (and more): http://physrev.physiology.org/content/physrev/72/1/165.full.pdf
Including this gem: "Serotonergic neurons are exceptional in their regenerative capacity in the adult brain." There is still hope! :)