The concern was touching, but sometimes Remus got worked up over nothing - okay, over small things. It wasn't always as bad or likely to explode in their faces as Remus thought. But when Moony really got that way, it wasn't always worth it to fight it. Sirius often did on principle, but that look...Sirius generally knew how it was going to go.
The answer he got back about concern did not answer what he meant by that. Yes, Remus was always concerned, always worried, and that wasn't ever going to change. "That's not what I meant, worrywart," Sirius replied with a small kiss, "I meant that bit about being worried I won't really want you or some nonsense like that." Even after Sirius had tried to knock it into Remus's head that he liked Moony a lot, he'd replied 'probably.'