Albert Barnes on Matthew 23:8

Be not ye, etc. Jesus forbade his disciples to seek such titles of distinction. The reason he gave was, that he was himself their Master and Teacher. They were on a level; they were to be equal in authority; they were brethren; and they should neither covet nor receive a title which implied either an elevation of one above another, or which appeared to infringe on the absolute right of the Saviour to be their only Teacher and Master. The command here is an express command to his disciples not to receive such a title of distinction. They were not to covet it; they were not to seek it; they were not to do anything that implied a wish or a willingness that it should be appended to their names. Everything which would tend to make a distinction among them, or destroy their parity; everything which would lead the world to suppose that there were ranks and grades among them as ministers, they were to avoid. It is to be observed that the command is, that they were not to receive the title. "Be not ye called Rabbi." The Saviour did not forbid them giving the title to others when it was customary or not regarded as improper, (comp. Acts 26:25;) but they were not to receive it. It was to be unknown among them. This title corresponds with the title "Doctor of Divinity," as applied to ministers of the gospel; and so far as I can see, the spirit of the Saviour's command is violated by the reception of such a title, as it would have been by their being called Rabbi. It is a literary distinction. It does not appropriately pertain to office. It makes a distinction among ministers. It tends to engender pride, and a sense of superiority in those who obtain it, and envy and a sense of inferiority in those who do not; and the whole spirit and tendency of it is contrary to the "simplicity that is in Christ"

(x) "But be not" Jas 3:1