SPOILERS! SPOILERS! SPOILERS!
Obligation
He felt a release when Lindsay came back to the lab. Her presence put the smile back on his face, the swagger in his step and that was very comfortable to him.
Being in Montana with Lindsay made both of them realize just how much they cared for each other. The long stares given across the courtroom, the laying of her head on his shoulder, and the almost-kiss was enough for him to realize just what a good woman he had in Lindsay.
After returning from Montana, they both talked about keeping the relationship secret for awhile. When the time was right, they would talk to Mac and see what the policy on inter-office relationships was. Mac would say it was alright, but they both respected Mac and the lab and they would not do anything to compromise the integrity of the lab and the workers.
He gives a small smile to Lindsay as he walks into the IAB office. The looks that they share echo throughout the hallway. She knows, he knows. He gives her a wink, as the door is closed behind him.
Out of obligation he takes over the lab, but by choice he loves her.