Imagine that you are standing on the equator of Earth, facing North. Now imagine a friend is also standing somewhere on the equator -- not too far away, but a fair distance -- also facing North. Now, if you both start walking you'll find that you begin to move closer together. The distance between you shrinks as you walk, until eventually you meet at the North Pole. The "force" that is pulling you together is like gravity in that it is caused by the curvature of the Earth's surface as you travel across it.

With gravity the curvature is intrinsic to space-time itself and is affected by mass. Generally more mass means more curvature. So if you sit two objects in space, at rest with respect to one-another (but still travelling forwards in time), you will find they are pulled together and eventually meet in the same way you and your friend were pulled together and met at the North Pole.

You might ask why mass affects the curvature of space-time? That I don't know the answer to.