Protons have positive charge. Like charges repel -- the electromagnetic force.
We need to overcome this repulsion to have the nuclei fuse.
How do we do this? Energetic nuclei! How do we make energetic nuclei?
Particles at a given temperature will have a distribution of speeds (and therefore a distribution in energies):
And, in this distribution, particles have a most probable velocity and an average velocity:
Well, things aren't that bad. There is a high energy tail to the distribution. But this alone isn't enough. We need to look to quantum mechanics.
The Heisenberg Uncertainty Principle: It is not possible to know both a particle's momentum and position to unlimited accuracy. In other words,
So combining the high energy tail of the M-B distribution and the possibility of QM tunneling (which rises as energy rises), we get the Gamov peak: