Diesel never said that Yanek was his mate or destined one...
Also, we aren't sure if Diesel was in love with him or not. If you remember earlier, he was questioning if his feelings were really love or just canine instinct.
I think Diesel's questions were more towards Ein (like does he love Ein because Ein killed his mate, so by canine instinct was he attracted to the stronger one, or is it truly love that he feels for Ein).
I think it's more towards the feeling it's self. Is any what he felt is love or is it all canine instinct? Also, he never referred to Yanek as his mate.
But he did say that Ein killed his previous lover, to which I think refers to Yanek -seeing as to how Ein is being targeted by Yanek's father for his death.
Soooo.... Yanek seems like a decent guy, and I'm having a foreboding feeling that I'll have a heart break once we find out how Ein kills Yanek.
Like... Bitch, Diesel clearly stated before that Ein killed his destined one -or mate, or something like that. You can't tell me Diesel didn't love Yanek if that's how he described him.
（/TДT)/
My kokoro is gonna brokoko soon... I can feel it.