Parents always want to know, "When is my baby going to start to smile?" Babies usually start to smile around two months of age, and they smile because they are happy, and often they smile because they see something in their environment that they are excited by.
A little bit before two months, around six weeks, you might see a baby start to smile, and oftentimes it's also involved like when they pass gas, they make this little smile, but it's not really an emotional smile yet.
By four months of age, a baby starts to laugh. They like when you pay attention to hem, they respond to your laughter and they laugh back.
If a baby doesn't meet these milestones, like smiling or laughing, that can actually be a red flag, because those babies aren't showing any kind of emotional intelligence or emotional IQ, and that would be a developmental delay, and that's something you should absolutely bring up to your pediatrician.
Finally, I'd just like to add that there's a myth that laughing is a sign of intelligence. Laughing is not a sign of intelligence, but baby having emotions is a sign of intelligence. You want a baby to laugh appropriately or you want a baby to smile in an appropriate setting, but it is not a sign of how intelligent your baby will be as an adult.