Q: Fun vs. Ending Credits?
A: Both.
ULTIMATELY I want to beat every game I play, but, REALISTICALLY some games make me choke, so I just work on them year after year (Super Pitfall and Bayou Billy on NES, etc.)
I CERTAINLY PLAY FRUSTRATING and/or LAME GAMES in order to have fun/beat them. I get obsessed with games and I'll want to beat them (or, at least, get to the next stage), to the point of masochism. MASOCHISM IS FUN FOR ME.