Some find it puzzling that Jesus does not rebuke them for this request as such. Part of the answer could be that that request cannot be seen as altogether negative: they respond to a promise that has been made (Mark 10:30, Matthew 19:28), and they begin, in their imagination, to apply it, so that the question cannot be seen as just despicable ambition but can also be interpreted as an enthusiastic willingness to be at the front in assisting Jesus when he has become king and judge of Israel.
On the other hand, it is true that Jesus’ reaction to what is missing in their request (though that request was possibly well meant) is implied in his answer. Insofar as one wants to serve Jesus, he shows the way. Insofar as one’s own ambitions can play a role, Jesus corrects that by pointing to the kind of way that is required (suffering, serving). We could add, in the third place, that Jesus’ rebuke of the desire for honour could be better applied to all the twelve than just to these two (see Mark 10:41).1
38 Jesus said to them, “You do not know what you are asking. Are you able to drink the cup that I drink, or to be baptized with the baptism with which I am baptized?”