Deidara thought they were lovers.
They were lovers because of the way Itachi looked at him, with admiration and a certain gleam of possessiveness that the Uchiha always had for something that was his.
They were lovers because of the expressions. The simple things, like when Itachi held his hand to the side- ever so slightly- so that Deidara would know to hold it. The way Itachi's mouth curved when Deidara would burst in the room and yell, or when he got scolded by his partner.
Deidara knew they were, because of the way he felt so comfortable around the man... the way he could frown or (if he needed to) cry in front of him, and not have to cover his hurt with a smile.
They were lovers because of the way Itachi's hands would linger longingly, on his hips or his hair, and the way he wouldn't flinch as he used to when the blonde snuggled into his side.
Deidara thought they were lovers because of the way he knew when Itachi wanted to see him. They were lovers because of the way Deidara slid under the sheets at night, after the others had gone to bed, and the way Itachi's arms would slide around his waist.
Deidara thought they were lovers,
but Itachi knew the truth.
Itachi could never love. He shudders at the thought of ever being described as a 'lover,' even if the context suggests he is Deidara's. No, Itachi has seen and promoted too much hate to ever be a lover. He believes that eventually, Deidara will tire of him, and 'lover' is too much of a permanent sounding word to serve his purpose.
Itachi shakes his head as Deidara climbs into the bed and then kisses the blonde.
No. He and Deidara aren't 'lovers.'
They are simply in love.