This is a good point. Money can make otherwise good people do some wild shit.
True, but I don't even really mean it in the way of greed in particular. Although it is often a factor if you have bad judgment. But really, being close to business partner always creates problem one way or the other.
You can never plan for what life is going to throw at you. Maybe you make a bad investment, maybe one your kid is sick, maybe you have tax issues, maybe you have substance issues, maybe your wife really needs a job and we could find her something, maybe you want to buy a house etc...
It's not so much the money that is the issue (although, it most often is in a way still), but it acts as a catalyst. Because when someone feels like they have a legitimate need and they get told no, they have a hard time to accept it on a personal level. It feels like the right thing to do from their own subjective perspective.
And if they are only a business partner, you still have a chance to try and help in other ways, to have a leveled discussion to try and find solutions. Not that it works all the time, but you have more options, and you don't appear as evil.
If they are somehow more than just a business partner, they will often consider it betrayal. Something personal you're doing to them. And they will try and appeal to whatever relationship you have to try and force your hand. To you or to others. They will become vindictive to the point that the original worry almost becomes secondary.
But honestly, that's only a few things. Ego is also a huge thing, so is pure unadulterated stupidity. Which is a huge problem in family ran businesses in particular.
You can find yourself dragging around deadweight, you can also have a partner who uses the business as an ego boost, and just lords around without even caring about results. But then, you also have pure stupid. The kind where your brother manages to spend 15k on new furniture during your fist vacation in 3 years. The salesman was very convincing.