Because he's already diagnosed Golovkin and took GGG's best shots. He knows what GGG's power feels like, so he's in a better mental state than he was coming into the 1st fight where GGG's power was the stuff of legends. Canelo found out, 1st hand, it's nothing he cant deal with.
Canelo is also only another year into his physical/mental prime while GGG is another year removed from his. I don't see any adjustments GGG can make, he's going to be in come forward mode all night, and Canelo proved he can hit fight GGG at mid range and be successful, as well as off the ropes.
There's 0 way I see GGG beating Canelo in the rematch. I think Canelo will be too prime, too fast, too explosive, and too skilled for GGG to beat him this time around. I also don't think we will see Canelo on his backfoot nearly as much as we seen in the 1st fight.
His stamina didn't affect him in their 1st fight, so I don't think it will this time around either.