There is no answer to this question. Nobody knows why gravity exists, or why magnetism exists, or why the strong and weak nuclear forces exist. Nobody knows why we are conscious beings and nobody knows why the universe is the way it is. It is like asking why are the laws of the universe one way and not a different way. That is just the way the natural universe works. We can observe that there is such a force which exists between any two masses - called gravity. We can measure it, we can even come up with mathematical laws that it obeys. And then take advantage of these laws and solve problems in astrophysics. But as far as why is it the way it is (or why is gravitational constant the way it is) we just do not know.
so if "why does gravity exist?" is actually the question, then, as TPH says, "nobody knows" is the right answer. but then that would make the assignment meaningless. many have theorized about why things exist, but no one really knows. it is one of those questions that religion seeks to answer, for example, but even that does not do a good job.