Our 2.5 yo daughter has always been outgoing and loves to be around people, until recently. All of a sudden she started lowering her head when we get together with people she doesn't see very often and will refuse to say hi or be friendly. Sometimes she has even been a little nasty. We certainly don't want this to become habit for her but we're uncertain of how much we can expect from a child this age. Is she too young to expect (or attempt to demand) that she say hello? If so, at what age should we start disciplining this behavior?