Each generation back you have twice as many ancestors (unless you had some who were marrying their second or third cousins)...sooner or later there is bound to be someone you'd rather no one knew about.
I can give a 12-15 cases of where library patrons where I volunteer got downright angry with me when I helped them find facts about their ancestors that they weren't too proud of. My favorite was the names I was called by the lady where I disproved her DAR line.
I also will not help people find their biological parents anymore either.