After hearing from Jesus what was going to happen to Him, James and John came to Jesus. They were not there out of concern for the suffering He had to endure, but to ask Him for sharing His glory with them. All that James and John saw was glory, they totally forgot that He had to first go through humiliation and death. Tribulation leads people to humility, and one who humbles himself will be exalted. The cross is inevitable.
Jesus reminded His disciples not to follow the examples of the Gentiles, that whoever wishes to be the greatest should not lord over people, but to be the servant of all. Seeing it from the world's perspective, why is being the greatest desirable, if one has to be the servant of all? They miss the point that those who are humble love God through their service to others. God will reward these people with joy and peace, even when they are still on earth.