Sae Itoshi wasn't someone known for letting his emotions get the better of him, on the contrary, he wasn't someone who even wanted to feel them on a large part of occasions.
So, Sae hadn't the slightest idea why, when in all the years of his life he had never felt anything close to romantic affection for anyone, he had started to feel it around you.
Maybe you were messing with his head or his sanity, and he couldn't deny that you had your charms, both physically and in terms of personality; he was just being objective when it came to admitting that you were a person he could tolerate on a permanent time frame.
The problem, he concluded, wasn't his feelings for you or whether you reciprocated, the difficulty came from the fact that you were already dating someone else when Sae met you, none other than one of his teammates; the only good thing he had found in those idiots.
He should feel ashamed for harboring such feelings for someone who already belonged in a relationship... but if he were honest, he'd be lying if he said he actually regretted the many times he imagined you as his.