They're still in school. Sure, some people find the love of their life in school, but others meet a good first boyfriend/girlfriend. They enjoy their time together, but grow apart and move on. It's life, it happens. I sincerely doubt anyone except Bill and Fleur will be getting married in book seven; while I figure that (if they live) Ron/Hermione and Harry/Ginny will be the 'ships JKR ends with, there's still a lot of room to grow.

*sigh* No matter which way it went, there would have been sections of fandom who were smug and satisfied. As it was, there were portions that claimed JKR was just wrong.

Thus, the only pairing I will argue for in HP fandom debates is Petunia/Snape. OTP all the way, baby!