why must three be an attractive force to explain the existence of slable nuclei?

Answer 1

The nucleus contains only positive particles and neutrons, neutral particles. There should be some attractive force, otherwise everything will blow apart.

Answer 2

You probably know that opposite charges attract and like charges repel. Now think about an atomic nucleus...it consists of a bunch of uncharged neutrons and positively charged protons. In an atomic nucleus, you're pushing together a bunch of positively charged particles whose natural inclination is to repel each other. In order for the nucleus to hold together, there must be some attractive force that overcomes this electrostatic repulsion of the protons.