Final answer:
The fusion of hydrogen requires high temperatures and densities because hydrogen nuclei have positive charges and repel each other. High temperatures provide the nuclei with enough kinetic energy to overcome repulsion, while high densities increase the chances of collisions.
Step-by-step explanation:
The fusion of hydrogen requires very high temperatures and densities due to a couple of reasons. Firstly, hydrogen nuclei have positive charges and repel each other due to electrostatic forces. In order for fusion to occur, the nuclei need to overcome this repulsion and get close enough for the strong nuclear force to attract them together. The high temperatures provide the hydrogen nuclei with enough kinetic energy to overcome the repulsion and collide with each other, while the high densities increase the chances of these collisions happening.
For example, in the Sun, the high temperatures and densities at its core cause hydrogen nuclei to collide frequently and undergo fusion, releasing tremendous amounts of energy.