Theophylact of Ohrid

Because then our Lord had said, He who is least among you all, the same shall be great, John feared, lest perhaps they had done wrong in hindering a certain man by their own power. For a prohibition does not shew the probitor to be inferior, but to be one who thinks himself somewhat superior. Hence it is added, And John answered and said, Master, we saw one casting out devils in thy name, and we forbad him. Not indeed from envy, but to distinguish the working of miracles, for he had not received the power of working miracles with them, nor had the Lord sent him as He did them; nor did he follow Jesus in all things. Hence he adds, because he followeth not with us.