Because everything Shin has been saying and doing ever since he found out he was an agent has been so unexpected and surprising that he feels deeper excitement and fondness for him everytime Shin continues to prove he’s made for him.
Re-reading the whole thing helped understand that lol. I don’t think I would’ve have without that. So the sweat drop in that one panel is really about him falling deeper for Shin.

wait I’m scared why did Jake have a little sweatyness when shin said he’d do it whahahah I’m so freaking scared it’s weird seeing Jake anxious…,,,