I think probably some of both, re. familiarity / richer depths. There's almost always something I notice on rereading, even for the umpteenth time, that I haven't noticed before -- and even when there isn't, rereading a book I like is like listening to a song I like, or eating a food I like. It's enjoyable regardless of whether or not I've read/heard/tasted it before. Sometimes the familiarity is specifically what I want, though, which I guess is the literary equivalent of comfort food?

And favorite first-time read this year, hmm. Prooobably the Hunger Games trilogy. With close runners-up being the Inkheart trilogy and Extremely Loud and Incredibly Close. And special mention for Out of the Silent Planet for being so endearingly wrong on the details and yet so passionately joyful about the idea of space travel.