Hummm.... well to attempt to answer the question a bit more fully:

Everyone knows that electrons are attracted to protons. If you recall your Rutherford description of the atom, you will know that the core contains these protons and some neutrons, then surrounding this is a cloud of electrons.

It is clear and proven (only through observing it) that the electrons are drawn to the protons. However, why do the protons not repel each other? What holds the atoms core together? These questions, as far as i know, have not yet been answered, although they think that there is a force which comes into play when these tiny bodies get extremely close.

Okay so back to gravity. Mass attracts mass. This is because in order to reach stability, atoms are attempting to reach states of lower energy (full outer shells, etc.). On a microscopic level, an atom with a larger posative charge than a negative one will have an overall posative charge (now it is known as an ion). This ion will attract atoms with a negative charge in order to react and achieve a lower energy state.
Now imagine this on a macroscopic level, and it becomes clearer why mass attracts mass.

If you haven't yet deduced that I don't know **** about this, and that I am just elaborating on a basic A-Level knowledge, then you are very easily convinced. I don't know perhaps its right, but don't go quoting it in any essay!