I can't agree that it came out of nowhere. There was definitely a slow build up starting in season 5 that Worf and Troi were becoming close friends. But if you really think about it, it never went any farther than that. The "relationship" moments never actually happened (Parallels, Eye of the Beholder). The closest they came was the kiss that almost happened in All Good Things. Even in Genesis when Wof gives her the "love bite" it's only because his body is transforming, it's not really him.
I think it was a realistic progression, though. Two people forming this makeshift family where she served as the mother figure in his son's life. In that situation it's only a matter of time before people start thinking about the what ifs. But, it would have never lasted. There are too many aspects of Worf's personality that he could never share with her. Deanna was no K'Ehleyr, or Jadzia