system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it matter in the Sun's core can undergo fusion at 15 million degrees but our fusion reactors need to be 100+ million degrees?
gpt
Fusion reactions are a slow/low probability process. In the sun, the time it would take for a nominated atom to undergo fusion would be significant, but this is offset by the sheer number of atoms present. 

In a lab based fusion system, the number of atoms in the pool is a lot smaller, so this is offset by increasing the pressure/temperature and increasing the chance of a nominated atom colliding with another and undergoing fusion.