Sure, Leonard knows that come hell or high water, Mick will always be there. It's a thing that's been true since they met and he doesn't see it changing any time soon, if ever. But there's always the chance that he could be wrong. So hearing the confirmation? Well, that's just icing on the cake.

"It's the way it should be--" A beat and then he cants his head just slightly to one side as a smirk slowly rolls its way across his lips. "Partner." There it is. The word they'd been searching for, for years now that actually fits in a way that isn't weird (like relationship) or undermining this thing between them (like friendship, but is perfect in all its simplicity.