On the Poverty of Seeking a Sign (from Christ)

The miracles, like the teaching, of Jesus were not offered as signs to convince spectators; they were done as acts of love to people in need. They only became signs for those with the humility and openness to perceive and receive the meaning. This the Pharisees clearly did not possess, as their attitude towards him shows. (BST)

Thus points out the BST commentary on Mark 8.12. The Pharisees seek a sign from Christ and his response is to sigh deeply in spirit (the Greek word is used only here in the Scriptures). He is not here to be a magician and they are completely missing the point of Christ's miracles. They will receive no sign and he emphatically tells them so.