"Why do deaf people shout!?"

This question could either be an expression of annoyance or a genuinely interested query. Assuming first  of all that it is a genuine search for information, here are my views, which are based on my observations and experience rather than any formal research.

Firstly, I agree that deaf people often do speak loudly. Interestingly, if they are able to wear a hearing aid, they do then speak a lot more quietly and you can often tell whether or not they are wearing their hearing aids just from their own speech. So my conclusion is that deaf people speaking loudly are instinctively doing what they imagine other people need in order to understand them.

It is also worth mentioning that people do tend to mirror each other's behaviour to some extent. So speaking in a loud voice can - again instinctively rather than intentionally - be a tactic to get other people to speak more loudly and hence be easier to understand.

Now assuming that the question was actually an expression of exasperation rather than a search for information, I do sympathise to some extent. Living with a deaf person can be taxing. The only way forward, in my view, is to sit down with the deaf person and caringly talk things through. Explain that the deaf person is talking too loudly by normally social standards and have a few practice runs about what is and is not acceptable.