Bullseye.
Lol
or maybe he thought because Byul is dominant omega, they couldnt be together.
dominant ALPHA* OMG my brain is not working properly today
More like they had a "relationship" as kids (we're talking kindergarten level here) and ML took the role too seriously, so when MC moved on to his next "boyfriend" he probably took it too heart and it goes on until current time. At least that's what I thought based on ML's intro in the sequel, but on the recent updates, it seems that they remained friends up until primary school, so my bet now is that it had something to do with the revelation of their 2nd gender, and when it was revealed that they're the same type, ML just lost hope of them being together + he seems to be struggling financially too so another thing could be their socio-economic stats.
U could be on to something
The black hair guy likes byul and he started distancing himself since you know he thought Byul wouldn't accept his feelings, or maybe he confessed and Byul didn't notice it and laughed it off?