I am a huge Brenda and Dylan fan, as well as Brandon/Kelly, and, like others, it has nothing to do with just wanting to keep Kelly away from Dylan.
My problem with Kelly, though--I really can't stand her character 85% of the time, although I did like the B/K combination--is that she always is mooning after Dylan or Brandon. If she's with one, she's obviously bothered by the fact that the other isn't hung up on her, and sometimes she's with someone else entirely and gets upset that they're not chasing after her. My proof for this opinion is obvious in episodes such as the one toward the beginning of the fifth season, when Dylan gets back from Mexico after getting ripped off by Suzanne and Kevin. When Dylan has the encounter with Brandon and Kelly at the hotel (the one where Donna's a freaky debutante robot), Kelly goes to Dylan's house alone in the middle of the night to talk, and is offended when he's with another woman. (Even sends Brandon over to chastise Dylan for treating her disrespectfully...) In the sixth season, she's offended that Dylan would announce his engagement to Toni without "considering her feelings." (Never mind that she's supposedly madly in love with coke fiend Colin.) When Brandon is seemingly happy with Tracy, Kelly can't stop pining away for him. Then, as others have pointed out, the exhaust fumes from Brandon's Mustang are practically still wafting through the air when Dylan comes back and Kelly's mooning over him. Throughout her whole "relationship" with Matt (who annoys me, too), she's always checking out what Dylan's doing. Even Gina asks Kelly to give it a rest!
Ugh, she irritates me. She's just like Brooke on "The Bold and the Beautiful"...if every guy around isn't sniffing after her, she's annoyed.
what a great point I had never seen it like that! Kelly did always pine over one or the other! (btw I know that other people might have seen this and might think i'm slow but I havent seen all the episodes so how would I know)