How did that whole thing start, anyway?
A bunch of people claimed that the UK was being "run from Brussels" and had no say in rules that affected us, and suffered as a result.
David Cameron, PM at the time, eventually decided to call their bluff and take the decision to the people.
Unfortunately, not enough people took the referendum seriously enough at the start, assuming that "the people" would be sensible and not decide to leave. Anti-Europeans were ahead of the game, though, and got their propaganda in first, and without the usual controls of election rules (thanks to canny use of social media, and what was later proven to be an illegal over-spend by pro-brexit capaigners) - the sides were not split along party lines, so there was often little control over who said what. Brexiteers made dramatic declarations of how much richer public services would be, how much easier and more profitable international trade would become, and the remain side made the tactical error of making quiet fact-checks instead of bold proclamations.
The biggest demographic split was (and still is) by age - older people are more likely to vote, and older people tended to be pro-brexit.
Result: a "majority" that was actually only 37% of the electorate.
There have been reports that, since 2016, enough voters have died, and enough young people have both turned 18 and become interested in politics that a re-run of the 2016 vote would result in a bigger majority for remain, but that is, of course, impossible to test.
Biggest worries, in no particular order:
> Border controls meaning that fresh foods sit in lorries on both sides of the border for days instead of hours.
> All branches of academia lose the ability to collaborate internationally.
> We lose access to security information from the EU, and become dramatically more vulnerable to acts of terrorism & organised crime.
> International trade becomes prohibitively expensive as we are excluded from existing and future EU free trade deals.
> Finance industry leaves the UK for EU offices.
> Manufacturing industry becomes prohibitively expensive (for instance, some parts of Minis, built by BMW, cross in and out of the UK/EU five or six times during the manufacturing process. After brexit, every single crossing could incur duties).
> etc