Imam Khomeini explains the matter as following:
If you are not able to reform and refine yourselves, and to manage and control your nafs al-ammarah [the commanding soul], to subdue your selfish lusts and to cut off your relations and interests with this world and material things, then after the end of the month of fasting it will be difficult for you to be able to accomplish this.
Therefore, take advantage of this opportunity before the magnificent grace of it vanishes, and purify and reform yourselves.
Get ready and prepare to perform the duties of the month of fasting.
Let it not be that prior to the arrival of the month of Ramadan you are like one who is wound up by the hand of Satan so that in this single month when the satans are enchained you automatically busy yourselves with sin and deeds opposed to the orders of Islam!