I can give a couple of reasons why a tree would be private. In my case, I was contacted by a woman a few years ago who turned out to be a close cousin, but I never knew that side of the family existed. She shared many wonderful things with me, and I know she would with anyone she determined was a relative. But she will never make her tree public. And I had things from her in my tree. Made me very reluctant to share it with the public. Other people think Ancestry is profiting dearly from our research, and don't want to contribute to that, but they WILL share with anyone they determine is related. Still other people have things in their tree they don't want made public, and have not yet figured out how to make just certain things private. There are many reasons, but to a person, that I have found, they will share readily with someone who they feel is related, or is polite enough to spend two minutes sending a note to them to discuss it. After all, you are probably only working on one person at a time. If you find a promising link and it's a private tree, how hard is it to send a note, like this one, when you may just end up with a new branch on your tree that is very important?