The factors which detract from Zeal are those which promote laziness. The greatest of these is the desire for bodily repose – aversion to exertion – and the love of pleasures to their very limits. There is no question that a person laboring under the above deterrent will find Divine service a great burden. For one who wishes to take his meal with complete relaxation and repose, and to sleep without being disturbed and to walk only at a leisurely pace, and so forth – such a person will find it extremely difficult to arise for morning services or to curtail his dinner so as to pray the afternoon service before nightfall or to go out to perform a mitzvah if the time does not suit him. How much more reluctant will he be to rush himself for a mitzvah or for Torah study! One who habituates himself to these practices is not his own master to do the opposite of these things when he so desires, for his will is bound with the bonds of habit, which becomes second nature to him.
From Mesillat Yesharim [ Path of the Just ], Moshe Chaim Luzzatto