The talker may assume the louder my voice the more they’ll understand what I’m saying, or maybe they feel like you’re not listening so they have no choice but to shout.
Nobody likes being shouted at and so the listener instead of focusing on the message will focus on the fact that you’re shouting and probably even be closed off to even trying to understand you.
People associate being shouted at with having done something wrong, it’s something associated with parents and teachers.
There’s not much use in shouting to try and get someone to listen, try simply talking instead.
But always remember, no matter how important you think your message is, you can’t force people to listen.