If Elena is sired to Damon because damon's blood made her a vampire, Caroline is also turned by Damon's blood,so is that also means that Caroline is sire to Damon too ? cause i didn't really understand this sire theory.
I think Elena is sired to Damon because they had a strong connection even when Elena was human. she loved him. Elena listened to him even before she was a vampire so the loyalty became stronger. Caroline was compelled by Damon so I think deep down she hated him so wasn't grateful it was his blood in her system. This is just what I think. :)
I agree with all of you: It is a random thing (like Damon once explained considering Tyler). But here is a not so serious theory: Caroline has an "anti sire bond"; she always disagrees with her sire, doesn't like him at all, does everything that she can against his will and so on. She has WAY more of that than she claims that Elena has in the other direction [I don't know how much "sire bond" DE have - or if any at all -, but Elena is still herself and can disagree with Damon].