These are the things that pull at Grantaire: The beauty and decay of the world. The endless cycle of history. Alcohol. Enjolras.
They are all linked together. He has always known himself to be star-crossed. He has always been something better before, will always become something better again. It is always there, an ache in his teeth. He exists to be part of another, their separation only temporary.
When he meets Enjolras, he falls into equilibrium, and he knows. When they perturb each other, he knows. They have no future but with each other.
They cannot hold it off forever.