It's that moment of thought that convinces Dirk, more than anything Bro says. It's that moment, because Bro is taking a moment to think about it. He's using his head, not just his heart. The answer isn't 'but I love him' or 'but he loves me,' it's that they're making it work.

"Long as you're sure," Dirk says. "Just needed to make sure you weren't a stereotypical schoolgirl about this shit. Bringing dishonor on the name Strider."