Harry was like a bird, Draco knew. He had to make a nest, a home, the place he loved above all others. He'd made that home at Hogwarts, which was why he'd come back to teach.
He also had to have someone to nest with, and Draco was thankful that he'd also returned - as Potions teacher.
But, like all birds, Harry would only return if he was knew that he was free to fly away.
That was why Draco knew he had to let him go.
He'd return. Eventually.