Nero's Links
Nero first met Smith in his human form and proceeded to make a fool of himself by flirting with him (Nero went full owo senpai notice me mode). Nero found out that Smith was a very high ranking warrior later, and he now lives in fear/shame of what he did.
Smith is a big dumb, so he didn't even realize Nero was flirting with him when they first met. He just took it as some weird dude being weird. But as soon as he realized what Nero actually did, he was at a point were he couldn't ask about it anymore without him looking like an idiot.
It has been awkward between them ever since.