I love science, and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awed by nature. Science is not meant to cure us of mystery, but to reinvent and reinvigorate it.
If a rat is a good model for your emotional life, you're in big trouble.
On an incredibly simplistic level, you can think of depression as occurring when your cortex thinks an abstract thought and manages to convince the rest of the brain that this is as real as a physical stressor.
Depression is not generalized pessimism, but pessimism specific to the effects of one's own skilled action.
Most people who do a lot of exercise, particularly in the form of competitive athletics, have unneurotic, extraverted, optimistic personalities to begin with. (Marathon runners are exceptions to this.)