Why do we start Ramadan with high vitality and then it fades away gradually?

Every act of worship has its form and content, so if we are concerned about the form of the ritual act of worship only, leaving the essence, this fading of vitality takes place. On the other hand, if someone tastes the sweetness of being close to Allah, his vitality will increase. Your diagram line is either ascending or descending, so if the performance of the act of worship is fruitful, it will ascend. It is like going somewhere and eating delicious food; rationally, you will go there again and again, but if you get there and eat nothing, you will not go there anymore.
When you do not achieve through your Siyaam the purposes of this month manifested in closeness to Allah, sincere repentance and tasting the sweetness of faith, you will, out of boredom, go back to the way you were before Ramadan.