Steve is worthy to lift Mjolnir because he is worthy of power
So this theory suggests that the reason Steve Rogers is worthy to lift Mjolnir is more than his goodness or morality. It's that Odin thought that the only ones worthy of power were those who knew when to put it down, or to share that power with others they trust by delegating or working together. It's an essential part of being a good leader, and Odin would only want someone able to do the job of ruling Asgard to access the powers of the mighty Thor.
It makes a lot of sense. As Dr. Erskine said, Steve spent most of his life with no power, so he understands both power and compassion. Steve has been a good leader, and he's also shown he's willing to trust other people's judgement, such as when he let Natasha make the call on whether Clint was free of Loki's spell. It also makes sense that Tony had no chance of lifting Mjolnir. His disrespect of other's time shows he's willing to wield his power over others for his own convenience. Even his misfortunes in Afghanistan didn't really leave a lasting impression, because he comes back and treats people exactly the same. Tony has never been a leader and never worthy.
Note: Set in an alternate universe where Thor didn't let Tony go near the scepter.
***
Tony was still angry about not being able to "study" the scepter. How could Thor know Tony's plans? Tony didn't have a clue, but he DID know that Thor flew away with the scepter and put it somewhere Tony doubted he could find in the next few days.
Now the team was gathered around the coffee table, discussing how to find the freaky twins who'd tried to disrupt their raid on the bunker. Maria, Clint, and Natasha were checking their sources, while Steve and Bruce were reviewing the records about the experiments Strucker had been performing with the scepter. That was supposed to be TONY and Bruce's task, but Tony had actually been sulking the entire time instead.
Tony looked at the hammer casually sitting on the table. He said, "Yo, Astral Viking, how many times do I have to tell you to keep your feet AND your tools off the table?"
Thor settled back in his seat and said, "You--and anyone else--are welcome to move them yourself, Stark. IF Mjolnir deems you worthy."
***
Thor made a remark about no one else being worthy, but he knew what he saw. Steven had pretended that he was unable to lift Mjolnir, but the hammer had responded to Steven's grip. Now that the immediate discomfort had receded, Thor was proud that his fellow warrior had proven himself worthy.
When Stark made another disparaging remark, Thor stood and said, "It is not a trick. Mjolnir can be lifted by someone who is worthy."
He then walked over and offered the hammer to Steven, hilt-first.
At Steven's raised brows, Thor nodded and offered the hammer again.
The room was silent as Steven stood and accepted the hammer. He swung with the same power and grace as he did his own shield.
Stark made a sound somewhat like an Earth chicken and said, "What the hell? So what is it, you need to be a goody-two-shoes from the Stone Age to take a swing?"
Thor shook his head and said, "Mjolnir is far more discerning. My father laid an enchantment that blocked me from wielding it until I showed I held the qualities of a good king. Worthy to lead the Nine Realms."
Thor sat down and gestured to the Captain. He said, "Steven understands power--its value and its price. To wield it wisely, a leader must know when to keep to yourself and when to to reach out. When and who to trust with power so you may work together. To always remember why you wield that power, and the people who are affected by its use."
He smiled at Steven's blush as the Captain handed the hammer back and sat beside him.
Thor gestured with Mjolnir and said to Stark, "These qualities and knowledge are things I have yet to see you begin to understand, much less demonstrate."
As Stark stomped off muttering about the system being rigged, Thor settled Mjolnir on the table once more. When he sat back, his hand brushed Steven's and Thor felt a jolt of electricity--and perhaps more--spark between them.