Uh, did the doc mention why the testosterone levels could be low to begin with? Diet / "life style choices" / stress / lack of exercise? At mid 40s simply 'age

Uh, did the doc mention why the testosterone levels could be low to begin with? Diet / "life style choices" / stress / lack of exercise? At mid 40s simply 'age' isn't all that convincing. What about side / ill effects?