Both fusion and fission are nuclear reactions. Why is a very high temperature required for fusion, but not for fission?

A nuclear fusion is defined as the reaction in which two small nuclei will combine together and result in the formation of a heavier nuclei.

As the nucleus of both the combining atoms are involved in a nuclear reactions. Hence, there will be electron clouds repulsion between these combining nuclei.

Therefore, these atoms will not be able to combine then. As a result, very high heat or temperature must be provided so that these electronic repulsion can overcome and tend to form a heavier nuclei.