I think that Stefan has no right to get insecure...he's the one who left Elena in the first place for saving Damon's life...he's the one who chose to be a ripper again...n now he punches his brother n demanding an explanation from Elena for that kiss...leave alone that i also feel that being with Damon for so long has made Elena more strong than she was when she was with Stefan...Damon has strengthened her as a person than Stefan ever has...Stefan does not love Elena enough n Damon loves Elena a lot...
But i also feel that if i have seen and understood at all, Stefan's character,then he will accept their relationship,coz even though he is a vampire,he used to be a human,n so that is y these insecure feelings...later on he will understand...
N as far as Elena is concerned,i also feel that she should stop trying to explain Stefan n herself n go ahead with accepting the fact,that she likes(or rather) loves Damon...