Majority of the scribes and the Pharisees were through and through against Jesus. They demand for a sign for what purpose? But Jesus knew them well; He knew the evil in their hearts.
We may not be like the scribes and the Pharisees who persecuted Jesus. But we are like them once in a while because sometimes we also demand for a sign from God. Especially when we are about to make major decisions we ask God for a sign.
For me, asking God for a sign is an indication of weak faith, come to think of it; why will we ask God for a sign if we have our full trust and faith in HIM?