Infants, toddlers, and small children all have one obvious trait in common: They have no idea how loud they squeal with glee when they are really happy.
It makes perfect sense; if a bit of fun warrants a slight exclamation, then a lot of fun must be commemorated with a much louder vocal burst.
They also share another trait: The abject lack of awareness about the difference between doing so on a playground or backyard and doing so on an enclosed public space like a bus or plane or train.
And accompanying parents are utterly powerless—perhaps due to fatigue, and perhaps due to the realization of the futility of trying.
Such squeals are more tolerable than cries or sadness, as they are interrupted by laughter.