A human vessel, when first born, cannot contain the angel they were made for. If an angel tried to possess an infant vessel, the baby would explode almost instantly. You saw what happened to Nick. But vessels are vessels because of bloodlines, right? That's what Michael said. But he left out a crucial factor. And he didn't leave it out because he wanted to keep it secret. He left it out because even he didn't know.
The life of the vessel must parallel the life of the angel.
Think about it: if bloodlines were the only thing that mattered, either Sam or Dean could have been Michael or Lucifer's vessel. It wouldn't have mattered who said yes to whom. And, when Dean was in Hell, wouldn't it have been easier to get him to agree to be Lucifer's vessel? And then, if Dean said yes to Lucifer, wouldn't the angels have hardly needed to do any work to convince Sam that saying yes to Michael would save his brother? But that's not what happened.
The life of the vessel must parallel the life of the angel.
Let's start with Castiel and Jimmy Novak. Jimmy Novak was a man who would do anything for his family. He was a devout believer in God, despite having never met him. He was brave and loyal to the point of self-sacrifice, even when the outcomes of his actions hurt the ones he loved the most. Castiel, too would do anything for his family, firmly believes in the existence of his Father despite all evidence stating that He does not care, has demonstrated enormous courage and loyalty and sacrificed himself on multiple occasions, even when the outcomes were different from what he expected. They are identical counterparts. They resemble a perfect pairing between vessel and angel.
Now Dean and Michael. Dean grew up with a distant –but known – father, and raised his little siblings the best he knew how. He was dutiful; always had blind faith in his father. Dean was told by that same father to kill his brother. Michael, being the first and oldest of the Archangels (Archangels being the first and oldest of all angels), raised his siblings in his Father's absence. Michael had faith that he was following his Father's will, and he dutifully pursued it. Michael was given the order to kill his brother. However, Dean was not ready to be Michael's vessel as soon as he got out of Hell because he was lacking faith in God, in his brother, and in himself – until Zachariah showed him the life of Dean Smith and made him believe again. John and Adam, in addition to being in the bloodline, had events in their lives similar enough to make them just as suitable vessels as Dean would have been.
The parallels between Sam and Lucifer are more compelling, and have nothing to do with demon blood, strong as that plot device may be in Sam's life. The demon blood was more of a way to taint Sam's soul and make him an unfit vessel for Michael, leaving him all to Lucifer. Sam was raised primarily by his brother, with whom he was very close. Though at times it seemed like Sam was the favored son, he and his father often butted heads over matters ranging from trivial to pivotal. Sam was kicked out of his home of sorts for going after what he believed in. When it came down to it, Sam did not want to fight and kill his brother. Lucifer, too, was raised by his brother Michael, and constantly fought with his Father despite his favored status. Lucifer was expelled from Heaven for taking actions to prove that humanity was not worthy of God's love, which he believed very strongly. When the time came for the apocalyptic fight, Lucifer proposed that he and Michael reach an accord instead of killing each other. The only reason their angel-vessel relationship did not work was because Sam didn't want it to.
The life of the vessel must parallel the life of the angel.
None of the angels were ever told of this caveat. Gabriel suspected it, but thought it was a funny coincidence. And really, the parallels are not important for the angel to find its vessel, since they're psychically connected and drawn to each other like a piece of metal to a magnet. The reason I'm telling you this is to warn you: you're a vessel yourself. Someday, you're going to hear a voice, and that voice is going to empathize with you on nearly every aspect of your life, and you'll want to say yes. I won't make any sort of judgment calls for you, and I won't tell you whether or not to accept the offer. You do have free will, after all. I just want you to be able to make this decision with all the facts, which I never gave Sam and Dean. But I learn from my mistakes, and you deserve better. So do with this what you will, and don't look back.
Sincerely,
Chuck