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

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.