Honestly, it totaly depends on you. Peps give rep for helping them out, solving troublesome issues, also if they're enjoying work of others (resources used by you, although not made actually by you); to folks who spend time writing feedback. Next: just to spread it and be able to give the same guy again. Not unusuall is giving it just for fun, for elaborating given subject, posting honest opinion, making people laugh, showing emotions.
And of course, you give reps to mods for just showing up ; )