I have often wondered about Willow/Kenendy - that mabe it was a bit like Biyffy used Spike to feel alive in S6. Willow wanted to know that she could cope with a relationship - but maybe didn't want to be hurt - so chose someone not looking at the long term. Both Oz and Tara were hard choices for her - and as Buffy had suffered they both left her.

Don't know whether what I am trying to say makes sense - but that has always been at the back of my mind when I have thought of the relationship - Kennedy was someone to help her escape and get through the next day