Those words didn't make a whole lot of sense to Dexter on a good, sober day. He intellectually understood their meaning and what it implied when somebody told them to you, but emotionally, perhaps even fundamentally, it wasn't something that he'd ever actually experienced himself. And that's one of those things you really just can't fake.
His only real way of reacting to the idea was to smile. "You love me in the 'hey you're my best friend' kind of way? Or something more?"
Dexter didn't feel like reminding her of how he was. He didn't think he really needed to. But he didn't want to ruin this moment.
Instinct told him that the best way of reacting to that would have been to say it back to her. That he shouldn't have asked her those questions first. He hoped that saying it now would mean something. "Love you too."