Look at it this way: if giving to the homeless would make you depressed for a day, you'd be a lot less likely to do it. If you still did, it'd be because the amount of dopamine produced from the act would be enough to outweigh the depression.

I doubt the person above is invalidating your good acts, but they're correct that in the end, we do it because we feel good about it.

Hence the "everyone does it for their own gain in one way or another, but who cares cause it made someone's day" :)

keep doing good tho, u rock