The Eustachian Tube is also a passage way for germs from the nose to the Middle Ear cavity.
There is a down side to the Eustachian Tube. In adults, it is sloped downward (no pun intended) from the ear to the mouth. But in very young children, whose faces have not yet developed downward, it is almost parallel.
Children in addition, have immune systems that are not as well developed as adults, but like many professors, they are continually putting their hands in their mouths (in my case it's usually my foot).
This is a perfect formula for the development of a nose cold. If you want to see a dozen or more, go to any kindergarten class. The problem is that the germs causing the cold have direct access to the Middle Ear through the Eustachian tubes.
Aside from the serious pain, the ears may fill up with fluid which hampers the movement of the tympanic membrane and ossicles.