Basically she asks to marry him, and he says yes, but in a really flippant and unromantic way. When she calls him out on it, he reminds her that he changed her diapers, and the diapers of several generations of her family. It's not a simple thing to just fall in love with someone who's descended from someone you raised like a son.
And she agrees that makes sense, and a relationship between an immortal and a teenage girl is going to require some work. At least they're together again, and she can get cracking on actually winning him over.
TLDR; he agrees to marry her out of respect for her efforts, but tells her to her face that he's not in love with her yet since she's still a kid, and she accepts that.