On the road to becoming a great apostle, John had a lot of maturing to do. After all, he was the disciple who reported to Jesus that he and his fellow disciples had “forbidden” a non-disciple from casting out demons in Jesus’ name. Jesus responded by saying “he who is not against us is on our side.”
Shamelessly, John and James, together with their mother, also tried to wrangle spots of honor by asking if they could sit to the left and right of Jesus’ throne in His kingdom.
In answer to their question, Jesus asked them if they could drink from His cup. They answered in the affirmative but Jesus told them that, while they could share in His suffering, it was His Father's decision as to who would sit where.
Jesus knew they were speaking out of pride and ambition. So, He rebuked them by saying that those who would be great in His kingdom must be the servants of others.