I dunno if it was only me or what, but when i watch an anime the first time, for example, Code Geass, I am exited to know what will happen next. The matter is in reverse for the re-watch. I mean, you know what's going to happen next, you know the ending, and stuff. This is different for when I re-watch an anime on the propose of finding answer to things i don't understand, though, cuz i know the plot, yes, but i have a gap on my knowledge on the anime.
To conclude all the stuff, i think the only time(s) that a re-watch makes you happy is when you dont have a complete intel on the series. Also, comedies are also one thing that is boring to re-watch. XD