I hate to break it to people, but the animosity you see between fighters is 90% of the time just BS to help sell a fight. Not many adult professionals have real beef with eachother, and they spend a lot more time in the vicinity of eachother during fight week than you think, doing media in the same area, traveling in the same infrastructure, staying at the same hotels, cutting weight in the same area, sometimes even talking to eachother directly to negotiate things.
Your opponent is your business partner in a fighting contract, in which the more interest generated in said fight the more money you both make, hence it is in both parties interest to stir up shit (on camera) to get fans more invested in the fight. Thinking they're actually at war with eachother and its on-sight is WWE-level mark stuff.