I'm not going to pay it forward at the grocery store
Do you have a reason to feel obligated?
I'm the only adult man in my house and no one goes out of their way to help me. Why should I bother to do that for others?
Well because you're talking about paying it forward someone must have helped you with something, presumably the last person in line paid for some of your groceries?  I can get that it feels like no one is helping you out at other times in life, but you should think about how  much it would mean to someone else in the same situation as you. If everyone does that the world will be a better place.
Like... I'm going to pay for someone's groceries and he can pay for some kid's diapers if he wants to.
Well paying for everyone's groceries can be very expensive. It is unreasonable to expect someone to spend so much money if they are on a budget.  If you can, though, it would be a very kind gesture to help out a stranger.