Some people disliked the line, but I personally think “I’m not sorry I met you” is the best thing Elena could have said to Damon. Even if she chose him and told him that she loved him, etc. wouldn’t he still have doubts? If she had never met him to begin with, she could have been happy with Stefan (as was pointed out to him). But instead, she took away every ounce of possible doubt by stating that, yes, she’d considered how easy things might have been if she’d never met him- but she still has no regrets. Essentially, Elena implied that it’s always been Damon.