As Alberta said, people can become 'very precious' about their tree. If we just go back to our grandparents, look how many descentants there are from them alone. So if you go back to great and great great etc, the number of descendants becomes huge. So many of us are going to have the same ancestors and similar tree. And all are perfectly entitled to put these ancestors in their trees. I love it when I match with someone. I have gained photographs of ancestors and so have they.
Kmt,
It sounds as if you are being bullied by someone with the same ancestors. Please do not give in to them by losing interest. On the programme 'Who do you think you are', they took some celebrity's tree back to when they linked in to the Royal family and beyond. Even the Royal family cannot turn round and say 'Excuse me, those of my ancestors, and mine alone.'
If you feel it has tarnished your interest and enthusiasm, I'm sure that will only last a short time. They are your ancestors full stop.
All the best Ann