This was something that occurred to me, and I'm surprised that I can't remember ever reading/hearing any philosophy about it:
I wonder if it isn't generally a bad idea to go into business with somebody you have a close relationship with (family, lover, friend)?
It just seems to me that sooner or later there's almost bound to be a decision where what's good for business will hurt the relationship, and vice versa. And there's just no easy way out of that.
I'm sure there are many examples of it working for many years, but I think that the percentage where it has had no detrimental effect at all on either the business or the relationship is pretty small.