Jesus' question comes out of grief. Before He spoke, "he sighed deeply in his spirit." Barnes says, "His soul, his heart, was deeply affected at their wickedness and hypocrisy." Gill explains the sigh as being "on account of the hardness of their hearts, the malignity of their minds, and insincerity of their intentions," and says the Pharisees "had no view to come at truth by this inquiry, but to ensnare him." Matthew Henry hears the weight on "this generation, that is so unworthy to have the gospel brought to it." Signs had just been given in plenty, including bread for four thousand (Mark 8:1-9). The question asks why, with so much already shown, they still demanded more.

Jesus answered His own question with a refusal: "verily I say unto you, There shall no sign be given unto this generation" (Mark 8:12). Then "he left them, and entering into the ship again departed to the other side" (Mark 8:13). Barnes clarifies that this meant no such sign as they asked, a sign "from heaven." In Matthew's account Jesus adds, "but the sign of the prophet Jonas" (Matthew 16:4). On the way across, Jesus warned His disciples, "beware of the leaven of the Pharisees, and of the leaven of Herod" (Mark 8:15).

"Sighed deeply" translates anastenazo, a word found only here in the New Testament. Thayer defines it as "to draw sighs up from the bottom of the breast, to sigh deeply." Barnes says "spirit" here means the human soul.

The Pharisees came "tempting him" (Mark 8:11). On the parallel in Matthew 16:1, Barnes explains "Tempting" as "trying him," that is, pretending to want evidence that He was the Messiah. In the Greek, "There shall no sign be given" reads literally, "if a sign shall be given," which the KJV turns into a firm denial. Matthew's account says the Pharisees came with the Sadducees (Matthew 16:1).

This question shows the heart of Jesus toward unbelief. He did not react with cold anger, but with a deep sigh. Matthew Henry writes, "He groaned (so some), being grieved for the hardness of their hearts, and the little influence that his preaching and miracles had had upon them." Jesus gave signs freely out of compassion, as when He fed the hungry crowd. He would not perform on demand for people who wanted to trap Him.

Matthew Henry adds a helpful distinction. When God spoke to particular people in particular cases, "they were encouraged to ask a sign, as Gideon and Ahaz," but when He speaks to all through His word, "it is presumption to prescribe other signs than what he has given." God had already given enough evidence. The issue was not a lack of signs but a lack of willingness to believe.