We don't know, but we can speculate. Based on the context, the most likely explanation is that
The Vision is completely pure of character, having recently been born with no opportunity yet to become corrupted or un-worthy. Like a newborn baby, at no point in his life has he experienced anger, deceit, selfishness, or any other "un-worthy" emotion. Unlike a child, however, he has an understanding of right and wrong, and has chosen to help The Avengers defend the innocent - perhaps even to the point of sacrificing himself.
Then again, we still don't have a complete understanding of what makes someone worthy. However, this theory is supported by dialogue in the film - in fact, it's the dialogue immediately preceding the event in question:
CAPTAIN AMERICA: Are you? On our side?
VISION: It's not that simple. (pause) I'm on the side of life. Ultron
This seems to indicate that The Vision does not see the struggle as "Avengers vs. Ultron", but rather as "Life vs. Death" - the purest of conflicts, with no room for moral ambiguity or variables. As such, his motive is equally as pure - to protect life.
The second prevailing theory - and one that may be supported by dialogue in the movie - is that The Vision can pick up Mjolnir due to his nature as an android. This is implied by Steve and Tony near the end of the movie, with the following dialogue:
THOR: He can lift the hammer, he can keep the Mind Stone. It will be
safe with him.
TONY: Well, he's not a man, after all. He's an android.
STEVE: What about an elevator? If you put the hammer in an
TONY: ... it still goes up.
THOR: Ah (laughs) I will miss our talks.
On the other hand, that first line by Thor would seem to support the first theory. Thor seems to imply that The Vision is a worthy protector of the gem because of his character... not because of his nature as an android. As it is, we are left to guess.