In order for the ear
to work properly air from the outside of the ear drum has to be balanced by air on the inside of the eardrum. In an intact ear drum the only way for air to get into the middle ear space is up the eustachian tube.
If the eustachian tube doesn't work well the air inside the middle ear gets used up creating a negative pressure. Eventually this pressure can become so low that fluid is sucked from the lining of the middle ear space.