To fans of Marvel Comics, the answer to the question, “how come only Thor can lift his hammer?” is “actually, whoever is worthy can lift it” (especially this guy), but also “magic.” The Marvel Cinematic Universe, however, is one that conflates magic and science as basically the same thing, opening up how Mjolnir bestows its worthiness to explanation in (highly theoretical) real-world terms. Physicist Jim Kakalios, writing for Wired, explains:
Most likely [the hammer] is taking some complex biological and psychological profile that calculates the “worthiness” of whoever is trying to lift the hammer. This is consistent with the scene in the clip where Steve Rogers (Captain America) is able to move the hammer (albeit slightly), while Tony Stark and Jim Rhodes, using thruster-assisted Iron Man and Iron Patriot gloves, are unable to budge Mjolnir at all.
Which is all well and good, but doesn’t necessarily explain why The Hulk can’t lift it. But Kakalios has an answer for that too.
When Tony Stark tries to lift Mjolnir using his Iron Man glove, he exerts a large upward force, greater than its weight, and yet the hammer remains at rest. So where does the additional downward force come from? One can only conclude that a unique property of uru metal is that, under the proper stimulus, it can emit large quantities of gravitons. On Earth, these fundamental particles have not been experimentally confirmed to exist, but as stipulated, the Asgardians are ahead of us scientifically. Gravitons are conjectured to transmit the gravitational force, and if an object emits additional gravitons, it is equivalent to increasing its mass. Thus, when an “unworthy” person applies an upward force, the uru metal increases the hammer’s weight to exactly cancel this lift, and the hammer remains unmoved.
So there it is. Gravitons and a combination of some kind of Asgardian nanotechnology determine whether a person is able to lift Thor’s hammer. There’s more at the link, including why Mjolnir is not made from neutron star matter (it would be too dense) and how the graviton theory could also explain the hammer’s ability to move mid-flight at Thor’s command—all of which is very grounded and scientific and not at all magical.