"The best of deeds are those which are the most difficult" (Usul al-Kafi, vol. iv, p. 9.).
For instance, waking up for the sake of praying to God Almighty in the biting cold of a winter night and sacrificing the delights of sound sleep makes the soul triumphant over the body and strengthens the will. Though it is a bit difficult and unpleasant in the beginning, but after a little practice, its hardship and inconvenience becomes lesser and lesser and the subservience of the body to the soul grows.
We see the people who perform it doing all this without any trouble, and if we are lazy and find it difficult, it is because we do not take action. But if we force ourselves to act, gradually the difficulty turns into ease.”
– Imam Khomeini (r), 40 Hadith (via al-sabur)