Because we as fans hate other players, we want our players to hate those players too, to validate our own hate. Unfortunately it doesn't really work that way. They're all ultimately part of the same brotherhood that we will never be part of, that of an NBA player. These guys do public events, charities, media appearances together. They may share the same agent. And even more so these days that everyone has cellphones and social media they're connected in ways that previous players never were.
But that's ok. I personally don't need them to hate the opposition, I just need them to beat the opposition. I'll do the hating from the safety of my couch 
The bolded is accurate, and I think it also applies to similar things. Specifically, I think a lot of fans want the coach to be an avatar of their own emotions. Throwing clipboards when the other team goes on runs, throwing players under the bus in the post-game presser, tantrums when the refs miss calls, getting ejected or picking up fines when the refs miss several.
There are valid reasons to be disappointed with Stevens right now, so this doesn't apply to everybody. But I also do think a lot of people struggle to identify with a guy who isn't as emotionally turbulent as they are, and that adds to their distaste.