I got the sense early on that they had a fling (or an almost-fling) in the past. Then for the rest of the series, there was sexual tension from time to time between the two of them (at least that was the vibe I was getting). So it was no surprise to me when they actually got together.
I go the impression (mainly from the episode Attached
) that Picard was always trying to hold himself back out of some sort of respect for his late best friend Jack. He felt guilty for having feelings for Beverly when Jack was alive and even after Jack died Picard felt like he was stabbing his friend in the back if he pursued a relationship with Bev. To be honest, other than Riker and Deanna, Picard and Beverly is the only Trek relationship that I would really think was believable. I guess Paris and Torres were fine, but I rarely ever think about them as a couple. I've never been a fan of Kira/Odo or Chakotay/7. Maybe Dax and Worf are okay, but even that felt a little off at times.