Disclaimer: I won nothing, I'm just a heartbroken Dair fan.
The idea of course came of what Penn Badgley said not so long ago about him thinking Blair was Dan's soul mate. He wasn't mistaken.
For it was not into my ear you whispered, but into my heart. It was not my lips you kissed, but my soul.
Dan knew Blair was his soul mate. He knew it when he realized how much they had in common. He knew it when he heard a knock on his door and every time he saw Blair on the other side, looking for him, and searching reassurance in his eyes, he wanted to give it all for her. "Everything is going to be okay, you're going to be okay." He always tried to tell her; if not verbally, he hoped his eyes matched his heart. He knew Blair was his soul mate long before he told her he loved her. Long before they started dating.
It surprised Dan how he could be so blind for many years and not notice this amazing, powerful, beautiful, independent woman that was right in front of him. He felt that the years before he loved her were a bad dream, fog, behind a veil.
But he was not delusional. He knew he wasn't Blair's soul mate, he was certain. He knew it when they held hands; when he squeezed her hand and she didn't respond or after a few minutes, pulled away. He knew it when, after a certain time of dating; her eyes didn't bright at his sight as his eyes did when he saw her. The illusion was gone, oh did he noticed it!
But what could he do? He did everything and it didn't change the fact that she didn't felt for him as strongly as he felt for her. He knew Blair was trying, and trying hard. And instead of reassuring him, her effort broke his heart. He wanted her to be happy above anything else. And she was not happy with him, or if she was, he knew she could be happier, someone else, her soul mate, could make her happier.
This is the reason why he knew he had to let her go. Of course, he was not letting her go to just anyone. Not any one will do for Blair Waldorf, his soul mate.
So when he started to notice the signs (he knew them by heart), he was certain that he could help Blair just this-last-time and let her go. And he did let her go. Blair, of course, was devastated, well, not devastated, but as sad as someone can be after a break-up. Dan knew one day she will thank him.
And she did, not verbally; she hoped her eyes matched her heart, her gratitude towards Dan. When he saw her again, she was changed, he was changed. They were both changed, and for the greater good as well. Blair was happy, radiant. She glowed so much that she lightened everyone who was around her, including Dan, even if it was only for the tiniest moment. And for Dan, that was sufficient, he could deal with that. He could deal to glow with her own light for the tiniest moment because he knew he helped. He could live with that tiny moment, cherish it forever and make it his own. Because deep down, still, Dan knew Blair was his soul mate.
So it is cleared, Blair's soul mate is not Chuck, God forbid. In my head, if she doesn't find love with Dan, she finds it with someone else, someone not involved at all with the UES. [Someone like her husband in my other fic: Conversations With Other Woman, who is barely described, but that is how I imagine him.]