Katherine is so very fond of telling him she loves him—that she always loved him—and a part of Stefan even believes this. This, however, is the ever hopeful, naïve side of him. The broody, realistic side of him wishes she was telling the truth.
But you don't compel people you love. You don't leave them for a century and a half under the guise of death. You don't threaten the lives of the people they care about.
And even back then, back when Stefan was human and Katherine was a mysterious beauty with an irresistible smile, Stefan had his doubts. He had loved her then, he will not deny that; but he had never swallowed her lies like Damon, never took her at her word and trusted that she loved him back.
He had looked for proof of her love and all he could find were memories of mind alteration, false kisses, lies, and a girl who slept with both brothers in the span of one night.
Yes, he loved her then—and maybe he loves her still—but he never believed she loved him too.
He just wishes she had—and maybe he wishes she still does.