Oh Kaylee. Tom really liked her when he met her that first time, with the free hug sign. He thought she was a special person even before he realized her connection to Dean and Sam. So I think he would have liked her and gotten to be friends with her even without that. Now she kinda fits in his 'family' on the ship. He's not really conscious of when it happened, but he's happy it did and he adores her and thinks she and Dean are cute and he kinda approves all around of the whole stability thing.

He feels BAD that she got possessed and a part of him wishes it was him instead, cause he's had to deal with that before and he's of the opinion that no one should have to experience that. Least of all people like Kaylee who are good and generous with their affection.

so, in short, he's kinda dopey little kid/big sister at her and really enjoys the fact that she's willing to put up with him dipping back into his childhood.