Desires and willpower (iccha shakti), both emerge from thoughts. Willpower is very important, like the life principle of every human-being. All other powers like power of intellect, of understanding, of determination, of speech, etc., come out of it. This willpower is like the king of all other powers. It will lead one to the sacred path if one makes proper use of it. On the contrary, if one makes improper use of it, the person becomes wicked. Willpower affects your thoughts, and thoughts influence willpower. Bad thoughts make the willpower weak. Decline in willpower makes desires stronger and ultimately weakens you! Here is a small example. Suppose one person has the habit of drinking too much coffee or tea, smoking cigarettes, etc. will his bad habits not weaken his willpower and ability to accomplish great things? Hence develop good thoughts, good habits and practice ceiling on desires.