He's surprised, but not as much as he could have been. It feels like a turning point in the truest sense - from here, they should fish or cut bait, and it seems like she's long ago made her choice.

So he says, "I love you too," as if they've been saying it to one another for years, as if it's the only natural response. It's why he took her dog, when she was gone. It's why he's willing to try this, even with her pregnant. Acting a bit mad is preferable to losing her. And that, on some level, is that.