If you're addicted to a game, it's only because it's fun, and you want to play more of it.
It's the same reason you go play another round of CS:S, or TF2. It's the same reason you level so much in Runescape. It's the same reason you play Blockland and read this forum every day.
It's fun.
The people who aren't addicted to it don't find it as fun. That's fine. It's not that a game is bad because it's not fun in one person's opinion, or even many peoples' opinions.