After Waterloo, there was no French army to help Napoleon continue to resist the Coalition powers. The main part of the army had melted away at Waterloo, Grouchy was nowhere to be seen, the Allies were pushing onto Paris, and the other French armies near the Pyrenees and the Alps were too far to help so Napoleon had no way to fight back.

Most importantly, I think that people forget is that Napoleon is realist. Once a Jacobin, he turned away from radical ideology after the Thermidorian reaction when he was placed in jail simply for being a strong Jacobin, from here he would turn away from extremist politics and become far more practical.

Further, there was no incentive to keep on. The Prussians wanted Napoleon dead but Wellington was willing to be more peaceful, so by surrendering to Wellington gave Napoleon a chance at life than simply fighting on to die at the hands of the Prussians. As I said, Napoleon was practical, and surrender was more practical than fighting on.