Here's why:
* Electromagnetic force: Protons, being positively charged, repel each other due to the electromagnetic force. This force would cause the nucleus to fly apart if it wasn't for the strong nuclear force.
* Weak nuclear force: This force is responsible for radioactive decay, but it doesn't play a major role in holding the nucleus together.
* Gravity: While gravity is a force of attraction, it is extremely weak at the nuclear level and plays a negligible role in holding the nucleus together.
The strong nuclear force is the strongest force in the universe, and it's short-range, acting only over distances comparable to the size of the nucleus. It's responsible for binding protons and neutrons together, overcoming the electrostatic repulsion between protons.
