A big reason is the rise of other great powers, particularly Germany. Before the unification of Germany, France and Britain were the two major powers in Europe. France was the most powerful state on the continent, while Britain's navy made it not only a power in Europe, but also around the world. As such, according to a realist view of international relations at least, these two states treated the other as their main rival. Then Germany united and started rapidly developing. All of a sudden there is this new state right in the middle of Europe that does become more powerful than France, and threatened to challenge Britains naval superiority. So they started adjusting their foreign policies to check Germany. Of course, it wasn't as simple as that. After WW1 Britian and France became the two de facto leaders at the League of Nations, but disagreements on how the new system should work and who should enforce it prevented them from working together to address the rise of Facist Germany. But in the end, it was the threat from Germany that brought the two countries back into an alliance.