As a kid, we were forced out of our coccoons all the time. Family gatherings, school, holidays, chores, scouts, whatever. We ran to our rooms to play games as an escape, a chance to be ourselves, by ourselves, with our toys. As often as not we'd have friends over, and we'd play games all day, all night, and all of the next day, until responsibility tore us away again. This is how I was, at least. I had a computer, I ran a BBS, and this was all very clearly my time, when I was finally free from social obligations.
As an adult I found myself hiding out more and more, simply because I could. I had money, lots of it at times, and nothing stopped me from buying all the games I wanted to play, staying home all weekend and playing them. But I moved around, I got married, I stopped having friends over most of the time; filthy buggers clogging up my controls and getting melted cheese in the carpet - bah!
So online and multiplayer games were a boon, a way to play with friends without leaving my comfort zone. And, to be sure, multiplayer games were always my favourite. Bomberman, of course, but also co-op games, racing games, puzzle games (Puyo, Puzzle Bobble). Single player games need to be so much better now, to capture my interest, 'cause no matter how fun it is I'm essentially wasting time alone, and I'd rather be with people.
Online gaming allows me to satisfy two urges: to be social, to play games. That said there's a definite paucity of genuine co-op 2P games. Some of the best gaming memories I have were of playing capture the flag online with a group of friends against a group of opponents. Quake nights, LAN parties - social gaming.
It's the way of the future!
PS: More co-op.