"The Lord was asked why His disciples did not fast. He answered that the time for them has not yet come. Then in a parable he showed that in general, the strictness of outer asceticism must be in keeping with the renewal of inner powers of the spirit. First kindle the spirit of fervour, and then take on austerities; for then there will be a new inner power capable of enduring them profitably. If you take them on without first having this fervour, because you were either impressed by the example of others, or wanted to make a show of your own asceticism, then it will bring no profit. You will sustain this austerity for a bit, and then you will weaken and drop it. And you will be worse off than before. Austerity without the inner spirit is like a patch of new linen on an old garment, or new wine in old wineskins. The patch will fall off and the rent made even worse; and the wine will burst the wineskin, and the wine will be lost, and the wineskin ruined. This, by the way, does not mean that austerity is bad, but only suggests that one must begin it in the proper order. The need for it must come from within, so that it might content the heart, and not just press from the outside like a weight."
Reference: http://days.pravoslavie.ru/en/Days/20110611.htm
