Black Bears 'aggression' is almost exclusively what is called bluster. It is a stress/fear response.
But they aren't social hunters so the ability to lose that fear response isn't something that was selected for.
Similar to racoons and even bald eagles, which are like pistons in Alaskan fishing towns, with a wider fear boundary radius.
Urban coyotes, which are typically solitary, mating pair or sibling group limited are similar, being there but hiding in the shadows in fear typically.
If you have ever had the misfortune of knowing someone with a wolf hybrid, that fear resulting in unpredictable behavior is fairly obvious.
Brown bears have more subtle stress indicators, if you watch the tragic videos recorded by Timothy Treadwell, and have experience with brown bear stress indicators it is quite obvious.
Obviously most animals are opportunistic, with even animals like horses being willing to eat voles or chicks that enter the pasture, and Treadwell stayed past the bountiful times when brown bears suppress their fear to co-feed with other bears which changed the calculus and ended poorly for him and his girlfriend.
The species that have been domesticated almost universally are cooperative either for protection or for finding food.
IMHO, the development of the ability to read human emotions is what allowed for the domestic dog.
My first job in highschool was as the kennel boy at a rural veterinary clinic. Dealing with injured animals safety was almost always about controlling my own lack of stress indicators and trying to communicate intent while observing for stress indicators in the animal. No matter if it was an injured dog or horse etc...
But you often had to remove them from their owners because dogs, being social are great at picking up clues from their owners. If an owner was concerned their dog would bite, it would almost always do so. Yet if you removed that cue, it is amazing how much less likely it was .
If you observe bears interaction with other bears it is typically filled with stress indicators like jaw clacking etc...
They never comfortable with any bear that isn't a mother or sibling.
So I doubt that there is a real reliable path to pet hood for them, just as there isn't, unfortunately, for most wolf hybrids.
The aggression (really almost always harmless bluster) is simply a secondary emotional response to the primary fear and anxiety.
They aren't 'thick' they just aren't built the same way.