I'd like to think there was some depth and underlying threads that the writers used regarding the relationship between Dylan and Kelly. However, I think it was more to add some soapy goodness and have a bit of a rivalry brewing (esp since minus Rique Brenda spend most of the time pining for Dylan).
In my mind most of the issues between Dylan and Kelly came because Dylan didn't really respect her. He didn't think she was intelligent or thoughtful. He seemed very attracted to her and I think he loved that she came with zero family drama. For Kelly, Dylan was a wonderful prize that validated her desirability. She had a long-term crush on him, didn't really date anyone in the beginning of the show (after suffering through her bad-girl reputation), and then had a very humiliating break up with Jake on Melrose Place.
Neither seemed very committed either, as they were quick to get their flirt on or even let the other assume they had cheated (like Dylan/Kelly in Paris during the summer, or Kelly with John Sears).
The "what about my feelings" rant from Kelly regarding the proposal is very true to form. She's extremely selfish and self centered. Plus she was just 21 (even if she looked 28). I can see how a girl who is that young would want some type of separate conversation with her "great love" about how he's moved on, still will love her, hopes she's ok with this decision.