At the end of Season 5 (https://en.wikipedia.org/wiki/Beverly_Hills,_90210_(season_5)), she turned down Dylan's trip around the world offer and Brandon's marriage proposal. What could be considered Kelly's selfish rejection, spurred Dylan to track down his father's killer. This led him to Toni, the daughter of the man who killed his father in the first place. Dylan at first, was using Toni to get information on his dad, but eventually they fell in love for real, got married and she ultimately got gunned down after her dad put a hit on Dylan to stop the wedding.

And as it turned out, Jack McKay (Dylan's dad), wasn't really dead after all. He faked his death and was in a witness protection program for 7 years.