This is funny, because we were talking about this as a family over the holidays. For friends of both my brother and I, who are 11 years apart, there was pretty much an open door policy. I had two best friends in high school, and I cannot remember how many times we would be out as a family, and come home to find one or both of them sitting on the couch, watching tv, and eating a snack. There was a designated snack cupboard, and about anything in the kitchen was a grab n go.

My parents surely had ulterior motives. They made our home an open door and welcoming place so that friends always wanted to hang at our house, and the folks could know what was going on. Both of my childhood friends who I am still very close to, but only see once or twice a year, usually see my parents more often than I do. They have family in the area. My female friend told me a while ago about going back to our hometown to pick up her daughter. She stopped at the folks to say hi, but noticed that there was a bunch of mail in the box, and figured they were out of town. She brought the mail in, got a soda, and fed the pets. =)

Just like everything else I suspect, a little bit of nature, a little bit of nurture. No one comes in my house without knocking because my dog has no social skills and acts like a dummy.