Here's why:

* Protons are positively charged particles.

* Like charges repel.

Therefore, the electromagnetic force between two protons is repulsive. This force is one of the four fundamental forces in nature, and it's responsible for many everyday phenomena, like electricity, magnetism, and light.

However, it's important to remember that within the nucleus of an atom, protons are held together by the strong nuclear force. This force is much stronger than the electromagnetic force over very short distances, and it's what allows protons to exist in close proximity despite their mutual repulsion.