I don't think he can, either. I feel like the perfect happiness that he had, somewhat, sprung from his naivete. He hadn't experienced enough sorrow yet to *not* be able to achieve perfect happiness. Even if he were with Buffy after NFA, I doubt he would be able to achieve perfect happiness. He's gone through too much. Well - maybe eventually he would be able too, but it would be a long, long time after NFA.