How do children learn to distrust people?

One thing that has been coming up for us lately is the repeated theme of getting our son integrated and comfortable being around other children.  Usually, August (our son), will either try to share something with the other child like a book, or his food, or some piece of string he found on the ground, but occasionally he prefers to just watch.  More recently, he and his Dad were with another Father and baby and that baby came over to August while he was sitting on the floor, and started patting his head – quite gently, at first.  But then gently, turned into not so gently – I’m gathering because August didn’t give that child the attention he was hoping for.