I have no problem buying Bitcoin from my friend. They are my friend for a reason.
It is easier than you may think to ruin a friendship by doing business with them. Not all friendships are the same, but I typically rather not do business with the closest.
Some friends are people you have known from childhood, who you have been through thick and thin together. Those inner cycle of friends I would talk Bitcoin with and buy Bitcoin from if they ever want to sell.
Also, many people change. Some for the better, others for the worse. It is likely not the first time you ever heard of friendships that ended really badly or people who used to be amazing turning exceedingly evil. Humans are in my opinion not trust worthy generally, even if they are the closest of your own blood. It often takes only a few triggers and they turn their back against you.