Why did not Trump shake Merkel's hand?

Today, US President Donald Trump told about the incident at a meeting with German Chancellor Angela Merkel, when he did not shake hands with her.

"Then someone shouted, ‘Shake her hand, shake her hand,’ but I did not hear it. You know, we've been in each other's company for a long time, and I had shaken hands with her for four times already," the American leader said, replying to the questions of journalists.