My sister-in-law gave birth to a fraternal twins, its only been 2 weeks since I noticed them smiling after feeding or changing their diapers. Their very cute and innocent, I feel like an angel is playing with them. Now, why do they smile? :)
I think it is probably an automatic reaction, a happy feeling makes certqain muscles and things in the body react to the feeling, I don't think many of us can really hide our true feelings.