Okay, I don't mean to be so negative but... what the hell was that?
Castle and Beckett are back together, proving once and for all that their separation was total nonsense. Nothing but a cheap narrative ploy to mix up things that did not need to be mixed up. Castle learns the truth about Beckett's plans to take down these evil drug people, and he confronts Beckett, asking her why in hell she had to break up with him to do this. Beckett goes on with some nonsense about protecting him, but Castle shoots down that argument just as firmly as I've been shooting it down all season - he points out that Beckett could have told Castle the whole truth and then they could have staged a breakup and took down the bad guy together. Castle says that Beckett is clearly broken, needing the obsession to drive her.
All of this would actually be a good point if Beckett's real behavior had backed it up during the season. But she "pushed Castle away" in name only, continuing to see him on a regular basis, and not fighting very hard against his attempts to fight his way back into her life. How does that indicate a broken person unable to form a good relationship? And how is that supposed to protect him from the baddies? It makes no sense. And if that's the explanation we're going with, why on earth did Castle forgive her so easily? It's pretty weak.