I think I ship Stefan/Caroline for the same reasons I ship Stefan/Katherine. Both Katherine and Caroline have a maturity about them (which I know sounds weird in reference to Caroline but here me out) that elena lacks. Elena often comes off as naive and much to vulnerable to influences to truly understand what she needs out of a relationship and more importantly, what she needs to bring to one. She is not nearly the interesting person she believes she is, and unfortunately that kind of thing comes only with years, experience, and life. Stefan needs a woman who can talk to him, who can be her own force in the relationship rather than simply a factor of his force, he needs a woman who can understand who he is and appreciate him for how good he is, but also recognize and not falter from his many flaws.
Both Katherine and Caroline seem to have a groundedness about them that comes from being in relationships that have failed, from being in love, heart broken and going through many of the experiences Elena has yet to really face. They’ve dealth with being the center of someone’s universe, and also loving someone more than that person loved them, and because of that not only are they interesting women, but they make choices based on factors they both understand are important in a successful relationship.
Elena is a child in many ways (look at her reaction to Damon’s death) and not nearly the woman Stefan needs right now. Maybe in time she will be that woman for someone, but for now I am fully on board with Stefan/Caroline (or more preferably Stefan/Katherine) for reasons stated about.