Every now and then I get to do someone a favor that primarily benefits me.  It's a win-win situation for everyone involved. 

My grandparents used to live in an area of Florida heavily populated by old people (a.k.a. anywhere in Florida).  Their neighbor behind them was an old guy with a screened-in pool.  Every time my family visited my grandparents, my parents would strike up a conversation with the neighbor, and I would get to swim in a luxurious, naturally-heated, 86° pool.  It was heaven.  And every time I swam in the pool, the old man would thank me for doing him a favor.  He said no one ever used the pool, so it was good to see it put to some use.  There's no victim in a situation like that. 

Oftentimes at large meal-gatherings, the host doesn't want a bunch of leftovers, so he/she will encourage everyone to eat a lot and finish everything.  On a normal occasion, I might feel bad about finishing the last of someone's home-cooked stuffing or freshly-made apple pie, but when I hear "No, please, finish it; you're doing me a favor," I comply.  I wouldn't want to be a bad guest.