Sometimes A baby is happy when they're smiling and sometimes the smiling look may be caused by gas buildup in the babies stomach.
From the minute a baby is born to one month of age babies do what is called neonatal smiling which is not for emotions at all.
The neonatal smiles in babies occur during the REM stages of sleep or when the baby is drowsy.
Some people think that a baby is always happy when they're smiling which isn't always the case especially until they reach one month of age.
As babies grow and mature they will smile less while drowsy or during REM sleep.
Also premature babies tend to smile more than full term babies.