Sure, testosterone gives guys the drive to compete for women and defend their territory, but it also helps them maintain a general sense of well-being, says Robert Brannigan, a urologist at Northwestern Memorial Hospital and associate professor of urology at Northwestern University Feinberg School of Medicine. Without it, your mood and motivation sink, and depression can set in. Even though women are more likely than men to be depressed, at age 65, those differences disappear. Some researchers believe that lower testosterone is the reason. Men with low testosterone who don't benefit from antidepressants might find relief with hormone therapy.