Damon only comes to her when Elena goes back to Stefan. Three hundred years down the line, and the pattern remains true. Damon messes up, Elena runs to Stefan, and Damon comes crawling back to her.
Caroline welcomes him into her bed because she can't help herself. He always looks so incredibly broken that she feels the need to comfort him. Between her and Damon, comfort only comes in the form of sexual gratification, and she can't deny that it truly is gratifying between them.
He often stays for less than a week. He drinks all her booze and hardly lets her wear more than some skimpy lingerie but she never kicks him out afterward.
She always knows when he's leaving. The sex is always particularly desperate and final on the last night of his stay. On the following mornings, she wakes up to silence in her apartment and a note by her pillow. It holds an address she'll never go to.
She tucks the piece of paper into her nightstand with the rest and cries into her pillow.
She calls Stefan that afternoon. "Can we go bunny hunting?" she asks.
Stefan never asks, but she assumes he knows. "Eventually he'll take too much from you," Stefan says to her one day, proving her theory, "and you won't be you anymore."
It's probably true, Caroline knows, but she convinces herself he'll eventually be kind enough to give it all back to her.