If you really love someone
Michael had heard of the saying 'if you really love someone, you let them go'. The idea being that if they really do return your love, they'd come back to you. To him, it sounded more like giving up and hoping (against all hope) that someone could read your mind. It was the stupidest thing he'd ever heard.
Why would you let go of all you had? Why would you let go of the only person who loved you? Michael was aware that Lincoln was no God. He certainly thought he was one at times but Michael knew, and he didknow, with painful detail, Lincoln's shortcomings. More than Vee or perhaps even Lincoln himself. Michael spent years observing his brother. Decades. Although Lincoln did unpredictable things, his reactions were fairly predictable to his baby brother.
He was where he was because of his brother and Lincoln was where he was because of him. It was that simple. As simple and as complicated as realizing that his brother loved him all along. That he also returned the sense of familial obligation. It was then he wondered why he was on the other side of the bars. The wrong side. He really loved Lincoln and he knew it without a doubt. So he would 'let him go' in a manner of speaking. Set him free that is.
As he walked towards the chapel, Michael held his head up and straightened his prison blues. He didn't give a shit about the saying, whatever way it was supposed to mean. If you really love someone you would go to jail just to be with them – then break the fuck out. Michael supposed the saying didn't sound quite so Hallmark and he allowed himself a smirk. When he saw his brother, he knew that no matter what Lincoln might think of his plan, he was on the right track.