So there’s a huge amount of buzz over this scene, and with good reason.
It’s already well known fact that only the worthy of Mjolnir are able to lift it. But it’s also been pointed out that Mjolnir isn’t merely heavy, but is more along the lines of stubborn. It’ll be lifted and used if it wants to be, so you have to pass its test.
Steve Rodgers, Captain America with a heart of gold. How could he not be worthy?
So here’s my impression; Mjolnir thinks Steve is worthy, but also knows that Steve wouldn’t really use it as effectively as Thor does, or if at all. So there’s no point in Steve wielding it. But Steve is one of the very few who is worthy enough, so Mjolnir acknowledges him.
Think of it as Mjolnir is a dozing cat that cracks open one eye to look at Steve before going back to sleep/ignoring everyone else.
(And this Thor hasn’t seen anyone else except for Odin lift Mjolnir, much less actually move it, so he’s shocked here and probably comes to the same conclusion; Worthy, but doesn’t need/want it. Plus, there’s the whole Ruler thing, and Steve is needed elsewhere, so…)