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.

Answering a "why" is a bit difficult when it comes to reality. It's just the way it is, and if it were different, you would be asking why it is that way instead, if you still existed at all.

If you're instead asking what causes the "force" you feel as gravity, then I suppose that can actually be usefully answered with the usual disclaimers about scientific models being only as good as their predictive power. Fortunately, one can accurately explain many phenomena with Newton's theories and even more with general relativity.

My own (layman) understanding is that the attracting effect of gravity is caused by the shape of spacetime itself; all objects within spacetime generally move "straight", but they must also conform to the shape of spacetime. However, those objects also affect the "shape" such that very massive objects have a macro-scale noticeable effect on the trajectory of other objects. and effectively what is "straight" is dependent on the local region of spacetime. I like to visualize it as a straight line on a piece of paper and how that line changes shape as the paper is bent.

We don't really know, even if we DO use math.

It's just a fundamental property of the universe that has no explanation (yet).

String theory tries to formulate why... but that's all just theory for now.