Newtons law of gravitation states that every body in this universe attract every other body. However, this does not mean that objects pull on each other, but that two objects attract each other with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them. It means is that for any two objects in the universe, the gravity between these two objects depends only on their mass and distance.

Regarding why pens kept on a table do not attract each other, as we understood above that the force of attraction is directly proportional to the masses of the objects, and you know that pens hardly have any mass. This probably is the reaason why they do not attact each other.