I'd say one of the main reasons is that on one occasion they got an uninteresting lesson. People start to insult mathematics when they can't handle the complexity of it, and have a focus on other parts of the curriculum.

I think the majority of people who don't like maths do so because they don't see the point of it. And to an extent that's true.

While lots of complicated maths is needed for all kinds of everyday things to work, such as pretty much anything electronic just for starters, people don't need to know it in order to use them. They don't need to know how something works as long as it does. And unless their job requires more advanced knowledge, most people could probably get by with no more than basic arithmetic.

And yet, everyone is forced to do maths until they're 16, by which time some of the things that they're learning are considerably difficult. Trigonometry, quadratic equations, and other things that they may well never actually need.

That's another possible reason. You start off learning how to add, subtract, etc. and that's perfectly fine. Until you've mastered that basic stuff there's really nothing else that you can be taught anyway. However, when you start to get more advanced then then everything you're taught is abstracted out of the real world and reduced down to just mathematical concepts and theorems. If instead of those, they were taught things that they could actually apply and use, then maths might start getting a lot more liked.

And of course, there are the added reasons of peer pressure (if you're the only one in your class who likes maths then you're weird) and the stereotypical view of maths being boring and for nerds, but those are just results of the previous reasons.