This generation is an evil generation;
it seeks a sign, but no sign will be given it,
except the sign of Jonah.
In all fairness, I think it should be noted that even though Jonah was a sign to the Ninevites, he was a very unwilling sign to be sure. Not only did he prefer to take a ship anywhere but Nineveh and end up getting swallowed by a big fish, even after this wonderfully successful preaching, he sat under a shade plant and lamented the mercy God had on the Ninevites.
But, that said, he was indeed a wonderful sign to the Ninevites. And they didn't require from him miracles and wonders. They heard his word – the word of the LORD – and reformed their ways, they straightened up their act. That's what Jesus is extolling here. It didn't take anything but hearing the word of the Lord for those evil Ninevites to turn to God for mercy. But the Israelites, who had in Jesus a much better sign than that of Jonah – a very willing sign for one thing – they still demanded a whole side show to test his words.
What about us? What does it take for us to make a change in our lives? Has God been trying to get through to us, but we keep holding out for some kind of sign? Shame on us when we do that – and most of us do at some point. We, like the Israelites, have a wonderful sign in Jesus, and we would do well to take up our own crosses and do what the Lord asks of us.