Why do Christians want to convert people to their faith even when their targets profess another religion?

Being a non christian (I am aTibeton Buddhist) many Christians upon finding out about me have professed their desire to convert me.. I know that they mean well and many have backed off respectfully when I told them that I am not interested but often many have after I have told them that I am Buddhist look at me in puzzlement. Why do you think they feel puzzled at me?