No-one knows what causes gravity. It could be just like the other forces with a graviton with spin 2, but I think there are hints that it is not: it is very weak (at our usual energy scale); it is exactly proportional to the inertia of an object, just like Coriollis force or centrifugal force.
If we understood inertia we would be closer to understanding gravity, but inertia is tricky. A body doesn't have inertia on its own, it is related to the rest of the universe.
A learned professor once told me that gravity was similar to the other forces "because there are four forces and gravity is one of them". So he didn't know either.