He sighed deeply and said, "Why does this generation ask for a miraculous sign? I tell you the truth, no sign will be given to it."
There is such disappointment in Jesus's sigh. Where there is no faith, there can be no miracles (Mark 6:5-6). Why? Because even if miracles, signs, and wonders were done, they would not be believed.
The LORD through the prophet of Habakkuk states, "Look at the nations and watch--and be utterly amazed. For I am going to do something in your days that you would not believe, even if you were told." (Hab 1:5) Miracles do not always lead to faith (John 12:37). This is just as Isaiah said, "Who has believed our message and to whom has the arm of the LORD been revealed?" (Isa 53:1) If the heart is skeptical to begin with, miracles, no matter how powerful, are explained away.
Counter intuitively, faith, no matter the amount, is required for miracles, not the other way around. Faith as small as a mustard seed can move mountains (Mat 17:20). If you cannot move mountains, it means your faith is not even as large as a mustard seed. Miracles only happen for the sake of those who will increase in faith because of them. This is why miracles are prevalent in countries like China and Nigeria because they are more willing to believe than in skeptical "enlightened" nations like America, England, and Sweden.
It turns out the more skeptical we grow, the blinder we all become.