Well, I've been with my partner for almost seven years (it's our anniversary this weekend, actually). Short-term, naturally, there was physical attraction and common interests but there was also that deeper sense of connection that's hard to put into words. A part of it was probably because we've both grown up in really dysfunctional families.
Long-term, the things that have kept us together have been real intimacy (a willingness to share feelings and be vulnerable and open up about the really deep underlying stuff), good communication (which takes a lot of work) and a willingness to try and work through conflict rather than just judging the other person or shutting them out.