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There are those more qualified to respond but here's my viewpoint.
Gravity is the name given to the attractive force generated by matter. The more the matter, the greater the force. So, I guess the real question is why does matter produce this force? Gravity is believed to act at the speed of light and possibly be messaged by a particle called the graviton. There are even experiments underway to see if gravity waves exist.
Locally, our measurements of this force say that it varies as the inverse square of the distance from a massive body. So here on Earth, Newton's Laws work fine. Massive bodies bend or warp space-time so that, on a larger scale, the laws need modifying according to Einstein's General Relativity. Observations of the speed of stars on the outer edges of galaxies have led to the dark matter hypothesis. However, dark matter may not exist. It is possible that gravity works differently at distance to what we expect and the equations need modifying (MOND). On a quantum level, we don't know how gravity fits in.
In Physics, theory can be generated from observation and measurement (Newton and the falling apple) and sometimes by modelling and testing predictions (Einstein and GR). String Theory has attempted to produce a quantum theory of gravity without success as yet. Maybe, we just aren't smart enough!
Regards, Rob
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