Health problems. Lots of things that can go wrong here but I wont bother to go into too much detail because you have heard it all before. These problems are very dosage dependent and drug dependent. Enlargement of the heart is probably the biggest problem. Although enlarging your heart sounds like a good thing, its very, very bad and is what leads to heart attacks in young bodybuilders. Damaging your kidneys and liver is another relatively common problem as the body works overtime to filter out stuff that it doesnt want. Then there is the problem with natural testosterone production being shut down.
The second theory is similar and is known as "evolutionary neuroandrogenic (ENA) theory of male aggression".   Testosterone and other androgens have evolved to masculinize a brain in order to be competitive even to the point of risking harm to the person and others. By doing so, individuals with masculinized brains as a result of pre-natal and adult life testosterone and androgens enhance their resource acquiring abilities in order to survive, attract and copulate with mates as much as possible.  The masculinization of the brain is not just mediated by testosterone levels at the adult stage, but also testosterone exposure in the womb as a fetus. Higher pre-natal testosterone indicated by a low digit ratio as well as adult testosterone levels increased risk of fouls or aggression among male players in a soccer game.  Studies have also found higher pre-natal testosterone or lower digit ratio to be correlated with higher aggression in males.