This is a a psychological phenomenon, when you are talking to someone you will create a bond—whether you like it or not. This will make you do things you may not be aware. Don’t believe it?
Let’s start with the more plausible things, did you notice that when you talk to a friend and give him something to hold, how he or she will take it? Well, this is the Situational Blindness. This works with complete strangers too, and it’s even more impressive.