Usually its because writers can't think of anything else. It's rare to see a platonic relationship on TV especially in teen dramas. That's my opinion anyway.

I wonder how long they would have dragged out the "will they, won't they" in "Bones" if Emily Deschanel hadn't gotten pregnant in real life.*shrugs*

I believe sexual tension can't exist without friendship as a foundation. But friendship can work without attraction. Chemistry is whats important. Otherwise it's just tension. I hope I make sense.*tries not to doubt my own theory*