The luxury of being an island.
It's not only that, it's probably also because of not having been occupied in WWII. Of course, that itself was in part due to Britain being an island, but it was also down to specific choices by Hitler (which, fortunately for the rest of the world, proved to be poor strategic ones) and Britain's responses to them.
Not being subject to occupation and subjugation by the enemy (on the mainland at least) as much of western Europe was, meant that the British felt little need for joining a postwar federation of states, and in turn, meant that Britons were more likely to think that being part of such a union was itself a form of subjugation. The only reason for Britain to do so was purely economic and that's how the Heath government eventually sold it to the voters - that the EEC would be good for jobs, trade and the pound in the pocket.
In reality, that was never the sole extent of the EEC/EC/EU project. It was always intended to become a much closer union, as the Treaty of Rome makes clear and subsequent treaties made that even more apparent. That disconnect between the stated aims and the way it was sold to the British voters was the foundation of that strained relationship and the strong Euro-scepticism, and as the greater aims became clearer, so did the tensions over Europe in the UK.
My personal view is that both the EU and UK are better with the UK outside the EU but being a friendly ally. The problem with the way that Brexit has unravelled so far is that the latter part - "being a friendly ally" - has not worked out yet. But hopefully, with the passage of time, cooler heads will prevail and both sides will settle into a relationship like two friendly next-door neighbours, rather than a household of squabbling flatmates.