Castle is not the same now as when he was with Meredith. Or maybe he's just a different man with Beckett. The episode "Significant Others" ends with Meredith revealing to Beckett why she and Castle didn't work out. It's an unsettling moment for Beckett when she learns it's because Castle couldn't open up. Immediately we're left to wonder, "Does he open up for Beckett?" The above moment from "The Wild Rover" shows, if it's important to her, he will share.
2. The Father at risk of losing his daughter.
Remember that cool (and hilarious) moment at the end of "The Wild Rover" when Ryan calls for his crew to step out—including a reluctant Castle incredulously asking, "Seriously?! I didn't... I didn't bring my vest"? That moment is the polar opposite of what we see when Castle is searching for Alexis in "Target." To find Alexis' location, a dark Castle says he "appealed to Douglas Stevens' humanity." He was reluctant. It's not a place Castle wanted to go. Now we know, if he has to he will.
3. The Boy so impressed he can hardly believe it.
Castle is someone who knows how to play. We see him do it all throughout "The Lives of Others." Shockingly, we also see that Beckett knows how to play, too. She plays him with... well, an elaborate play. When Castle discovers that everything was a setup for his birthday, he is, after a jaw-dropped moment, ecstatic and shouts that it is, "the greatest birthday present of [his] life!"
4. The Successful moments after the ultimate pursuit.
Everyone was dying to see this—how would Castle react to finally getting to be with Beckett? In "After the Storm," we get to see it. Pleased. In disbelief. Beyond happy. We like to think there was a special version of Castle saved just for this moment.