John Gill Commentary Daniel 10:19

"And he said, O man greatly beloved, fear not: peace be unto thee, be strong, yea, be strong. And when he spake unto me, I was strengthened, and said, Let my lord speak; for thou hast strengthened me." — Daniel 10:19 (ASV)

And said, O man, greatly beloved. Or, "man of desires", as before, (Daniel 10:11) , which shows it to be the same here speaking as there, and probably Gabriel.

fear not; for a man has nothing to fear, from men or devils, that is beloved of the Lord; and especially from good angels, how glorious and majestic soever they are.

peace be unto thee; all prosperity of body and soul; inward peace of mind, a freedom from all hurry of thought, and commotion of the passions, and eternal peace and joy in the world to come.

be strong, yea, be strong; take heart, pull up the spirits, be of good courage, play the man; be strong in the Lord, and in his grace, and fear nothing: the word is repeated for the greater encouragement.

and when he had spoken unto me, I was strengthened: divine power going along with his word; the prophet found his bodily strength renewed, his spirits revived, his heart cheerful, and his soul comforted, and all fear and dread removed from him; which was owing to the energy of divine grace; for otherwise not only men, but angels too, would speak in vain.

and said, let my lord speak, for you have strengthened me; and so was able to bear the sight of him, support in his presence, and hear his words, and take in what he said, which before he was unfit for; so an angel may be an instrument of strengthening a saint, yea, a prophet, and even our Lord Jesus Christ himself as man, (Luke 22:43) .