Allen never did what he did for the reputation or the gratitude. He did it because he was the only one who could. Initially it was also to atone for his sin, his ignorance. His selfishness.

"Yes sir." Allen said mostly for Link so he could go away and snitch back to that annoying mustache bastard.

Then Komui said a magic phrase. One that Allen should have prepared for and masked his reaction towards. However missing someone so dearly didn't help keep the bigger picture in focus. So naturally Allen ended up perking up in interest.