I think what Beckett is afraid of, ultimately, is losing Castle. If they get into a relationship, she could lose him because she’s not ready and she’s not sure he’s ready. She still has questions about the kind of guy he is. We’ve seen him grow up a lot in the last three years, but I think any woman getting into a relationship with a guy like this on that level would have those questions. And he knows how he feels about it. But given what he found out in the aftermath, he wants to do it the right way. He wants to have the right relationship with her. Deep down, he knows — whether he’s expressed it to himself or anyone else in the universe — he knows he’s met this person who he can spend the rest of his life with. He just doesn’t know how to go about it because he’s never done it successfully.
— Andrew Marlowe, EW interview (via lcrell