system
Given the following post, respond as if you're commenting on Reddit:
human
Why is nuclear fusion so much harder to achieve than fission?

Edit: thanks for all the replies guys, I understand this a lot better now!

Edit: Solved
( commented but my flair didn't get added :3 )
gpt
Many heavy nuclei like to undergo fission on their own, producing neutrons (among other things). Neutrons, being electrically neutral, have no problem smashing in to other nuclei because they aren't electrically repelled from the protons. Being hit with a neutron causes nuclei that like to undergo fission to break apart immediately, rather than just randomly; control your environment so you get many neutrons hitting other fissile nuclei and you achieve a chain reaction.

Fusion requires combining two nuclei, which are both positively charged. It requires a lot of energy, in the form of temperature or lasers or explosive compression or whatever, to get two positive charges close enough together so that nuclear forces take over. Doing it in a bomb is easy enough, but doing it in a controlled situation is a much harder problem.

In short: fission requires slowing down neutrons so they hit more nuclei (easy), but fusion requires speeding up nuclei so they hit each other (hard).