John Calvin Commentary Mark 8:12

"And he sighed deeply in his spirit, and saith, Why doth this generation seek a sign? verily I say unto you, There shall no sign be given unto this generation." — Mark 8:12 (ASV)

And groaning in his spirit. By these words, Mark informs us that it caused our Lord grief and bitter vexation when he saw those ungrateful men obstinately resist God. And certainly, all who desire to promote the glory of God and who feel concern for the salvation of men should have such feelings that nothing would inflict a deeper wound on their hearts than to see unbelievers purposely blocking their own way to believing and using all their ingenuity to obscure with their clouds the brightness of the word and works of God.

The words, in his spirit, seem to me to be added emphatically, to inform us that this groan came from the deepest affection of his heart, and that no sophist might allege that Christ resorted to outward displays to express a grief he did not inwardly feel; for that holy soul, which was guided by the zeal of the Spirit, must have been moved by deep sadness at the sight of such wicked obstinacy.