He thought he was in love with his brother and that kept him from being direct. He didn't even notice he was in the picture and even if he had, how would he know Souichi was keeping the picture because of him when he's so young in it? Souichi only had that picture because it's the only one he could find, but because he was hiding it and because Yuuta was so young in it, it looks like he loves his brother. Besides, the same thing can be said in reverse, if Souichi thought they were dating, why no confession, why no "i love you"s, why no dates, why no communication at all? They both had a responsibility to talk, since neither of them did, that cancels out, but the picture thing kept Yuuta from making a move and that's Souichi's fault for hiding it and making it look suspicious. He's ignoring those feelings because it was Souichi at fault.
Is hard for me to be on Yuuta's side. The misunderstanding for years wasn't even Souchirou's fault to begin with, and everyone points at him for being blockhead. If you know he's like that, why not being direct with him? Then he makes an issue out of nothing. Has Yuuta considered the feelings of Souchirou? xD who thought they were a couple for years. Who's ignoring whose feelings