AFAIK, the reason still isn’t well understood. You’re going to get a lot of speculative answers here, but that’s all they are.
Recent speculation seems to center around an inflammatory response. Science vs. did a podcast on it not too long ago as well.
[https://gimletmedia.com/shows/science-vs/39hz2e6b/hangovers-what-really-helps](https://gimletmedia.com/shows/science-vs/39hz2e6b/hangovers-what-really-helps)
Added: This TED article summarizes a lot of the confounding issues.
[https://ideas.ted.com/the-science-behind-hangovers-and-what-to-do-when-you-get-one-david-nutt/](https://ideas.ted.com/the-science-behind-hangovers-and-what-to-do-when-you-get-one-david-nutt/)
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