I've had that happen to me too!
Not only does it get on my nerves to have someone come up to me and touch my hair without permission, but I don't think anyone has the right to scold me for wanting to keep it. I've had people ask me if I was going to donate it (that's fine), or tell me flat-out that I should
donate it (not so fine), that it would grow back, and don't I want to help someone out? Well...maybe if it was my own idea, and if I didn't have a horrible track record for getting bad haircuts! Either way, selfish or not, it's my