'Stefan's not coming back. Not in your lifetime.' I mean, she's young, but human beings only live maybe eighty years. For Stefan and Damon, that's nothing. Unless Klaus rips their hearts out, they're going to live for another thousand years."
Stefan observing Elena for weeks and deciding to stay in Mystic Falls because he “had to know [her]” is three steps beyond messed up and into psycho-stalker territory.