Few things can make us so happy when we see our baby laughing for the first time. The heart is almost gone and we run to find a camera to preserve such a memory forever.
Then, they do it again, and they obviously want to decorate our day with that cute, happy face.
But what's most interesting of all this? According to a study done at the University of California, babies laugh at their mothers because they expect the same facial expressions. This kind of communication is made simply because they are happy to see mom laugh! 
Babies do a certain kind of behavior when they see their mothers smiling, but not every smile gets the same response. Toddlers only return laughter when they see mothers are 100% focused on them and the smile is dedicated to them.
For example, if the mother is laughing and turning her head away from her son / daughter the opposite happens. They do nothing because they didn't get the attention they were expecting!
This is in line with another study that shows that children learn by imitating adults, especially mothers.
