Harry thinks that Severus is only "nice"/interested in him because he is Lily's child. This could really be the truth... or not. ;)
Severus is only strong on the outside, but inside he feels undeserving/ugly/whatever. Harry needs to teach him how desirable and wonderful in his own way he is.
(And my favourites, memory loss and time-travel/de-ageing have already been said. :) )