The feeling you have when you've done something you worked on.
let's take a first 1cc. it doesn't even matter which difficulty it is, can be easy, normal, hard or lunatic. first 1ccs are hard work and never come out of nowhere, but once you've managed, it's all like "wooooooooooow, awesome". you know? I remember my first 1cc lunatic. I was freaking happy - and that was only TD.
Same with the perfects I've been doing the past weeks. Some of them were hard work and took a lot of tries (I.e MoF stage 4, MoF extra or IN Stage 6A). And even though most were done pretty quickly, it still feels awesome to have perfected Koishi, for instance. It also looks cool.
that's probably the main reason why I'm playing.