I have an UshiTen-centric Ushijima headcanon which is Ushijima is actually very aware of when people are romantically interested in him, and it is because he knows Tendou is insecure and he refuses to indulge that voice in his head.
In my mind, UshiTen start dating their first year at Shiratorizawa. Ushijima regconizes his feelings towards Tendou aren't the same as his feelings towards Semi and Reon, so he asks them about it. They have long discussion where they conclude Ushijima is in love with Tendou. Semi and Reon blink and Ushijima is already down the hallway ready to confess to Tendou.
Once UshiTen start dating, Ushijima quickly learns Tendou is insecure and while Tendou loves him back there is always a small voice in his boyfriend's head from his younger school days saying he isn't enough and Ushijima's popularity can exasperate it. Afterwards, Ushijima is hypervigilant about people's romantic interest in him, and because it is Ushijima his rejection is very blunt.
Cue Shirabu joining Shiratorizawa as a first year with a crush, only for a month into training to be on the receiving end of one of the most mortifying talks you could ever imagine where his crush tells him to move on sooner than later because it is never going to happen. Shirabu is tough so he is able to take it, but afterwards Semi and Reon have to sit Ushijima down to tell him to be a little less harsh.
Tendou is happy, and that is all that matters to Ushijima so he'll be as harsh as he likes.
Until Goshiki joins and Ushijima has to be nicer about letting him down because Tendou is particularly fond of him. It still isn't the most graceful and Goshiki can't look Ushijima in the eye for a month, but it is better.
