If you asked him (I mean, you'd have to tie him up first and hook him up to a lie detector with a gun to his head, and then ask him) he would probably tell you that he guessed it had began the day at the diner. The day when they were transporting the perp, and she had talked constantly, complaining that he was antisocial and uncooperative. He busted out the hambulance and the henway for two reasons.

One, so that she would shut up, which he would have told you point blank.

Two, (and you'd have to dig the gun into his temple here to get him to admit it) because really, he missed her smile. He hadn't seen that smile in- what was it- days? Weeks? At least, he hadn't seen that smile directed at him. At Nash, at Diaz, at (he would try not to grimace at this part) Callaghan, but not at him.

Though he wouldn't admit it (even if you turned the safety off and fingered the trigger), he loved Andy McNally. So it makes sense that, when you love someone, you want them to be happy. Luke made Andy happy. He made her smile, so no matter what he wanted, he wasn't about to step in the way of that. He might act like a bad boy with a heavy rep, but really, he wasn't the kind of guy to come between two people who were well on their way to...well, where ever they were going. If Callaghan made Andy smile, than he's be damned if he stopped her smiling, even for a minute. Which is why he hammed (excuse the pun) it up for her in the diner.

He might be able to make her smile for a minute, in a remote diner, while in uniform, but everywhere else, Luke was the one to create that response. Andy was happy (at this point, you'd drop the gun and start blubbering, trying to untie him through your tears).

To Sam, that was all that mattered.