Prayer, as such, doesn't scare me, in fact, I'll even ask for it, on occasion...But
I consider my faith to be an ongoing, loving, and personal conversation between myself and the Divine.
It has been my experience over the years that when a stranger overs to pray for me without my asking, it's often because that person believes my own faith system is faulty or wrong, or that my life is on a wrong path (as someone who cannot walk on her own, a lot of strangers assume I am unhappy and want to walk -- I don't). And it's that judgement factor that really ticks me off.
It's like trying to have a private conversation with a loved one, and a stranger comes between us and says: "No, no. What she really meant to say is..."