Actually...
Testosterone increases regeneration rate of cells, and that's why men die earlier. However, castrated men die éven faster, because of a lack of testosterone. (Unless you're on hormones it seems. ^^)
Anyway; testosterone also decreases the effect of your immune system, while boosting your recovery rate. (Wonder why ill men always just go on with their life, no matter the illness? That's it.)
It's also known to dull nerves, especially pain receptors. As a rule, men are much more resilient to pain.. Well; they don't get as much of it as women do, because of testosterone. (Women in labour really don't count, as that's rather situational.)
So... It's not that strange that it also affects the brain, like that. The good part is: Brain cells actually dó regenerate. (Albeit very slowly.) The bad part: Male brains grow old a hell of a lot faster.
As for aggressiveness: This is actually due to the fact that 'fake' testosterone, and I mean steroids, simply isn't compatible with the human species. The behavioural changes are because the stuff triggers aggressiveness rather than defensiveness, and that's because it activates the wrong brain cells.
As another point of interest: How does killing bain cells compare with aggressive behaviour? After all; if that would be linked, then concussions would make people very angry. As far as I know, concussions from accidents simply make people whoozy, instead.