I see what you are saying, but I have a problem with that reinforcement being a kiss on the mouth. When had we seen previously Buffy kissing someone simply to reinforce good behaviour? To me it was too much to simply be that simple. She could have hugged him, she could have just spoken nicely to him, she could have just smiled kindly at him (he would have been happy with any little crumb) but she kissed him on the mouth.
I realise I am bringing my own bias into this, and obviously there are people who kiss others on the mouth to say thanks, and I do not. But I would also say I cannot remember Buffy ever doing that before. And all those things previously mentioned - a kind work, a sincere smile, a gentle hug, they can all instill pride in the receiver (even a playful punch on the arm) - a kiss is more , and means more, or I believe it does to Buffy