Anyone who is not familiar with hearing loss assumes that it is like turning the volume on a radio lower and lower until,you have difficulty understanding the words, or cannot hear at all.
Hearing loss is far more subtle than that.
A person with hearing loss usually has a different amount of loss at each frequency across a range of hearing. For example, a person may have normal hearing in the lower frequencies, a moderate hearing loss in the middle frequencies and a severe loss in the high frequencies.
In relation to hearing aids,and in order to understand speech better, the person wants the low frequencies unchanged,the middle frequencies a little louder, and the high frequencies very loud.
Increasing the volume of the radio makes all the frequencies louder. The volume is so loud in the lower frequencies,where the hearing is normal, that it becomes intolerable. Louder does not mean clearer in relation to frequencies unless the frequencies are properly balanced.
You can find out more here.