Hormones and bio-chemicals are greatly overlooked. As you got older you realize how much our your life has been prompted by biochemical signals. Feelings of appiness, fear, pleasure and lust are usually just the reponse to some chemical released by a tiny gland somewhere in your body or from somewhere in the depths of your brain stem.
Now take testosterone. It does all kinds of things for everyone. men, of course, more than women. It builds muscle mass, stimulates energy and hair growth. Men are just flooded with testosterone in their late-teens and early twenties, and then after we have done our little bit for genetic diversity, it's all down hill from there. Of course, civilized lifestyles have changed the timeline quite a bit but our genetic coding has never caughtup.
Anyway, the older you get the less testerone is produced with the obvious results. A friend of mine who probably would prefer to remain nameless and blameless, once espoused the theory that as the amount of testosterone a man's body produces decreases, there is an increase in the amount of another undiscovered hormone we can call the Cranky Man Hormone. The older you get the more of the hormone is produced. I was running around touting this theory, until my wife instructed me that this is a bad theory and a really lame excuse for being cranky.