Responsibility for another's life is a whole other kind of motivation that surpasses a great many obstacles. Unless you're a terrible person to the core. The biggest problem with depression isn't that it makes you feel terrible for no reason, it's that it robs you of the ability to feel pleasure for all but the most fleeting of moments. It takes more and more effort to feel any kind of pride, joy, or satisfaction, and those sensations become more and more brief, until you justify having water for dinner because eating would cost too much effort and money to be worth it.