Deidara didn't know why.
They had only one bed in the house. When Sasori was there, they slept together, in both meanings of the phrase. But Sasori was a professional artist. He was often called to exhibits or asked to present his work somewhere, and Deidara couldn't always go with him. He had a job, too, you know, so he stayed in the house. Alone.
They had only one bed in the house. It had green silk sheets, that they switched out often, as they had many just like it. On top of the sheets was a lovely royal blue comforter, that was folded and stowed away in the closet in summer. Deidara's pillow was light blue, to match his eyes, and Sasori's pillow was light green, because Deidara said it matched the sheets, and because Sasori never objected.
When Sasori left on business he would make it up beautifully, and often when Deidara came home from work he would find a letter or a present waiting for him, perfectly in the middle of the bed. To last the blonde while Sasori was gone, he said. Deidara appreciated it, and if there was something there for him he would lean over the edge of the bed, never touching the comforter, and he would snag it without disturbing the sheets, or the pillows, because Deidara liked it like that. He liked it just the way Sasori made it.
As time went on, he found he couldn't sleep in the bed while Sasori was gone. He would toss and turn, and close his eyes, but he could never sleep, and soon he stopped trying. At first he made excuses for himself, and instead of sleeping in the bed he would fall asleep on the couch, or on the floor (because the carpet was soft, and because Sasori wasn't there to tell him not to.)
There was just something different about the bed, something was just different. It was the same bed, and he knew, because long ago he had checked, he had ripped away the comforter and the sheets down to the mattress, and it was the same. He could never do that again. He could never bring himself to disturb the bed the way Sasori had made it.
He figured it out, one day, on one of the days Sasori came home from a trip. The bed was just as he left it, but Sasori would never notice. He would think that Deidara had just made the bed again, and Deidara never told him otherwise. He just silently climbed in next to his boyfriend, because Sasori was tired, he had been walking around all week, and if Deidara wanted to do something they could do it tomorrow, and Deidara didn't mind. He was just happy to be able to sleep right again, with Sasori, and the green silk sheets, and the royal blue comforter, and the light blue pillow that matched his eyes.
It was the same bed, he knew that. He had checked. There was just something different about it when Sasori wasn't in it.