many reasons:
-because he was drunk and forgot
-because he regretted it
-because he's shy and doesn't now know what to do/say
-because he feels too awkward after it like he can't know you/speak to you the same way because of whats happened
-because he likes someone else and doesn't want anything messing that up, so you were a mistake
-becuase hes a d*ck and just did it for fun in the moment and can't be bothered addressing it with you
-because it was a bad kiss
-because someone made fun of him about it