Well, here's the easy answer: because it has
mass! Our current understanding of gravity is
what's called Einstein's theory of general
relativity. This theory says that mass causes
space and time (combined into one thing called
spacetime) to curve and bend. So, a massive
object (like the Earth) warps the spacetime around
it. But when objects move, they move through
spacetime, and so a curved spacetime causes
objects to move in curved paths. But this curved
path makes it look like there's a force acting on
the object causing it to bend, and we call that
apparent force the gravitational force.
To paraphrase the famous physicist John
Wheeler, "Mass tells spacetime how to bend, and
spacetime tells mass how to move."
Hope these help!
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