Albert Barnes Commentary Daniel 6:21

"Then said Daniel unto the king, O king, live for ever." — Daniel 6:21 (ASV)

Then Daniel said to the king, O king, live forever. This was the common form of salutation in addressing the king. See the note at Daniel 2:4.

There might be more than mere form in this salutation, for Daniel may have been aware of the true source of the calamities that had come upon him and of the king's innocence in the matter. He doubtless recalled the interest which the king had shown in him when he was about to be cast into the den of lions, and his expression of confidence that his God would be able to deliver him (Daniel 6:16). Moreover, Daniel must have been favorably impressed by the solicitude which the monarch now showed for his welfare by visiting him so early, and by his anxiety to know whether he was still alive.