1. Respect for his captain
Zoro is Luffy’s first mate. Although initially he had no respect for his captain, Zoro began to witness what Luffy was capable of throughout the course of their journey together.
Zoro has a grand dream of becoming the greatest swordsman in the world but Luffy has an even grander dream of becoming the Pirate King. Zoro more than anyone else knows the difficult path that lies ahead for himself and even more so for Luffy. This is why he has immense respect for his captain.
Eiichiro Oda has excessively made a point that they are the only two people more than ready to sacrifice their lives to achieve their dream and has put the two on a pedestal in that regard.
This is a special bond that exists only between these two. Luffy living up to Zoro’s expectations is what makes Zoro loyal to him.
In short, Luffy living up to Zoro’s expectations and Zoro believing without question that Luffy can achieve his dream is what makes him loyal. Loyalty and respect for Luffy makes it a duty for Zoro to protect his captain first and foremost. In Zoro’s mind, if he is not even able to protect his captain’s dream, how the hell is he going to ever achieve his own dream.
2. It makes the most logical sense
If we take the time to think about the predicament Zoro was in, we can understand that he actually made the most logical decision he could at the time. Zoro was in a pinch, every singe one of his crewmates were completely knocked out in the fight against Kuma and the Shichibukai was a threat of impending doom for the crew.
Had Zoro stayed silent, Bartolomew Kuma may have completely wiped out the crew, which would result in the end for not only Luffy’s, but also of everyone else’s dream. If Zoro makes a successful deal with Kuma, he can secure the crew’s life and their journey to becoming pirate king.
3. What this means for Zoro
This was the biggest character progression for Roronoa Zoro. A man that was always driven by one goal willing to sacrifice himself only in the pursuit of his own goal, had turned into man that would sacrifice his own life for someone else’s dream.
The fruition of this scene is later shown when he practically begs Dracule Mihawk to train him which the Shichibukai points out is extremely pathetic, embarrassing and something he never thought Zoro would do. For someone that was willing to give up his life for his captain, discarding his pride wasn’t a difficult thing to do.