Why does the fusion reaction of light nuclei occur only at very high temperatures?

For the fusion of nuclei, it is necessary that positive nuclei overcome the forces of Coulomb repulsion and approach each other at a distance of the order of the size of the nuclei. for this they must have high kinetic energy, which is possible at a very high temperature – about 10 ^ 8 K
Temperature is a measure of the kinetic energy of particle motion. Such temperatures can be obtained, for example, by pre-using a nuclear reactor or using laser beams.