Impossible Faith

In Luke 17, the Disciples ask Jesus to increase their faith. Jesus responds by telling them: "If you had the faith like a mustard seed, you would be able to say to this mulberry tree 'be uprooted and planted in the sea', and it would obey you." Why did Jesus say that? Was He mocking them? They already acknowledged their need for more faith. So, why does Jesus seem to mock them for not even having the amount of a tiny little mustard seed? Jesus' meaning, here, is vital instruction about our faith.