The מסילת ישרים speaks of certain things that we know instinctively and others that need to be learned. Nobody needs to be taught the need to eat and sleep. A person will satisfy these needs without being taught. However, matters such as love and fear of Hashem are not automatic. One needs to find the means to achieve these cherished qualities. One must work very hard and receive guidance from holy people in order to get there. The Ramchal reminds his readers that one day when one reaches 120, he will be asked why was it that he spent so much time and effort in other endeavors, and did not put forth the effort to acquire love and fear of Hashem. Sometimes we need to be reminded of the obvious or otherwise the obvious could be forgotten or overlooked.