One day a friend of mine asked me "how's your day?" "Better than yesterday" was my answer, since I went through a pretty bad day the day before. That friend then said "so you are at a local maximum?" Then a discussion about maximum/minimum in life began.
Since a person can only be in one emotional state at any particular time, we can consider one's mood as a function in terms of time. If one is in a good mood, then the function would have a positive value; if one is in a bad mood, then the function would have a negative value. This function is only defined in the times when we roam this earth. This function is continuous, but can fluctuate wildly from day to day. We shall call this the mood function.
So how do we know when we are at a maximum/minimum in the function that is life? The easiest way is to compare how one feels at the present with one's emotional state in earlier times. In one's life, one experiences many emotion peaks(local maxima in) and valleys(local minima), but only one global minimum and one global maximum (there may be someone who has more that one global maximum/minimum, but that is highly unlikely).
Of course, some may say that one won't reach one's global maximum/minimum until one no longer walks the earth, but there probably is not anyone on earth who can record this. That is why I only define the function in the time between one's birth and death.
I quipped that it would be nice to be at my global maximum at that particular moment. After this remark was made, another friend of mine joined the discussion. He said that if one's emotional global maximum is in the past or the present, then one would have nothing to look forward to anymore, since there is nothing left in that person's life that will match the high one experienced at the global maximum.
Clearly, there is no way for anyone to know at what time one's mood function is at the global maximum/minimum until we know the entire function. That means that we have no idea of knowing when the global maximum/minimum occurs until we are facing death (I know, I am being morbid here). Since we don't know when our global maximum is, it probably is a good idea to stay positive and hold the belief that one's global maximum has yet to come. If one believes that one's global maximum is in the past, then one may feel discouraged and that one would have a bleak future. It would be difficult for them to feel good, which would bring the value of one's mood function even lower. Then one would feel worse, and one's mood may continue to slide. And this may become an endless cycle.
Yes, this is a real life application of calculus (this must be a first). It is also shows that sometimes simple conversations can evolve into discussions about life. Anyway, still positive. Your global maximum may or may not be in the future, but at least you can keep the values of your local minima positive by staying in a good mood.