Testosterone

It is all about control, excessive testosterone develops a greater need for control over your environment ie. you have a greater competitive attitude to all elements with in your environment, human and otherwise. Angry faces are there because you are creating them, forcing a change in the emotions of others around you and specifically forcing them to adopt negative unhealthy emotions and stealing away their ability to control their mental state ie. challenging them and winning.

...but isn't this classic Alpha behavior? Do the psychologists even study chimps and gorillas any more? If your underling (or a potential threat to your rule) only briefly shows an anger face but not long enough to be perceived as a threat, that means you 'won' the confrontation and have established your superiority in the heirarchy. Of course, with chimps it's actually the ones that stay on the fringes and avoid the Alpha (we'll call them Chimp Geeks) that have the most sex with the females, as the Alpha & underlings are too busy and stressed about staying on the top o' the heap...

http://science.slashdot.org/science/07/05/15/2026249.shtml