Why do "new-born" babies smile/laugh ?

Newborns don't smile socially until around 6 weeks or so. But they do smile earlier than that, and it is usually due to some internally pleasant sensation. Around 6 weeks they start using smiles as a way of showing happiness to see their primary people and as a reaction to fun visual stimuli.

Reflex more than anything. They don't smile in response to anything right away, but they do spontaneously smile and often laugh as well.

The whole "newborns can't smile, it's gas" way of thinking is completely outdated.

They don't laugh, they do with looks like a smile but it is justthe way they look when they pass gas.

Agree with CJ.

Even though a baby has a lack of intelligence of the real world and life. They are still over whelmed with uncontrollable emotions and see a new world which is pretty much an excitement.

Because they're happy they get to see the real world instead of a deep dark place with limited movement.

They suck on nipples all day. I'd be smiling too

Gas.

They do this when relieving gas most of the time.

they cry... not smile...