Natural or UnNatural

There are many things in life which are quiet natural, but considered highly unnatural or disgusting. Best example is a three letter word. But I don't want to discuss it now as it is so much over hyped now a days. Let me stick to simple things :-)
When you are sleepy its natural that you yawn. But you are supposed to say sorry for that. When you are full with food you burp. But again you have to say sorry. The same applies to the other 'embaressing' sounds (;)) produced by our body. There are things which are highly unnatural, like a smile plastered on a nervous face, or the sound of high heeled footsteps or the stupid piercings on different parts of the body. But these doesn't require a tag of 'sorry' along with it. I don't know why we have to apologize for things which we are born with and not for the ones we added on later.