Nuclei are made of protons and neutrons, so every nucleus has a positive charge. Two positive charges repel each other. This is called electrostatic repulsion, and it gets stronger the closer the nuclei are.
For two nuclei to fuse, they must get close enough together for the strong nuclear force to bind them, and that is a very short distance. To get that close, they must overcome the electrostatic repulsion. At low temperatures and pressures, the nuclei move slowly and are far apart, so they repel each other and bounce away. Fusion does not happen.
At very high temperatures, the nuclei move extremely fast, so they have a lot of kinetic energy. They can then collide hard enough to get close together and fuse. A high pressure squeezes the nuclei into a small space, so collisions happen more often. This is why fusion happens in the core of a star, where the temperature is millions of degrees and the pressure is huge because of gravity. Fusion of heavier nuclei needs even greater temperatures, because their charge, and so the repulsion, is bigger.