Should I ask my friend for my money back?

I few days ago I lent my friend $20 so he could buy himself some dinner because he didn't have any, and I thought he would pay me back but he hasn't yet. He is kind of irresponsible with the way he spends money, including other people's, but I was just trying to do the right thing. Should I keep asking for him to pay me back, or should I let it go? I mean, $20 is kind of a considerable amount for me, but I don't want this to get between us.