you could say you have an interest in the subject for personal reasons (this could mean for yourself, or for a friend, or for a family member).

Another thing you could say is that you've explored resources for the subject, and you've talked to people experiencing this kind of thing etc.

Mention that you honestly care and want to help. Maybe that you're acceptant of lots of different types of people? Something that indicates your interest level and knowledge level.

You could even say someone in your family struggles with depression and you've learned alot from them - it would be true because YOU are in your family

*just throwing out ideas - might not help but meh xD call it brainSTORMING*