The gaming industry is increasingly making games more social. People are now getting more and more used to playing with strangers online. The whole idea of achievements and badges definitely enable this. Yes it's great to beat a game, but now you can beat a game and then boast about it publicly - feeding his/her's ego and that is immensely addicting.

Diablo 3's DRM is practically invisible, because you need to be online anyways, how else would you boast to your friends about that new legendary item that you just got.