Everyone wants a taste of being at the top, whatever you call it. The curiosity drives them mad. In this movie it is the top of the lighthouse. A symbol. At first people are happy they got a chance to work for that special symbol. Then they would want more, until they eventually want to own it, exclusively.

They can't stop thinking about it until they go insane and desperate. Every little detail feels like part of something bigger conspiracy. They would even kill for it. This desire is like a disease, devouring, rotting the soul at its root. And then after all the time and efforts spared, the sacrifices and struggles, they manage to climb to the top. But just like any other lighthouse, with merely a bit of extra brightness within the structure, the paradise is never to be found there. It essentially is simply another place to be, of which the meaning was given by other people, who you don't even like in the first place.