[Actually... hang on, just a second. It's true that the other team members have tried to hide their relationships in the past. JJ tried for a year with Will, and it really didn't work despite him not being around all that much. Kitty would live here, most likely, which means she'd be around all the time. How did he plan to keep that a secret, and even more importantly - why?
Because that's what he does. It's what he did with his mom, for obvious reasons, and the dilaudid dependency and pretty much every single thing that's ever happened to him. He keeps it secret best he can. Separates his personal life from his professional so well that the people he's worked side by side with for years don't even know where he lives. In some ways this is good, to protect the people you care about, to create a sanctuary - but that doesn't apply to Kitty.
Most of all... all the details he's been hiding about himself up until now have been nothing but bad. She... she's the complete opposite. For once, he's got something good in his life. Something he shouldn't feel the need to hide.]