Jamieson, Fausset and Brown on Daniel 10:2

2. mourning--that is, afflicting myself by fasting from "pleasant bread, flesh and wine" (Da 10:3), as a sign of sorrow, not for its own sake. Compare Mt 9:14, "fast," answering to "mourn" (Da 10:15). Compare 1Co 8:8; 1Ti 4:3, which prove that "fasting" is not an indispensable Christian obligation; but merely an outward expression of sorrow, and separation from ordinary worldly enjoyments, in order to give one's self to prayer (Ac 13:2). Daniel's mourning was probably for his countrymen, who met with many obstructions to their building of the temple, from their adversaries in the Persian court.