Based on Bhagavad Gita Chapter 14, Text 23
As aspiring devotees, we will often observe the state of our consciousness oscillating up and down like a sine wave. In our devotional service, we feel sometimes attracted towards Krishna, sometimes apathetic or even averse. These feelings correspond to the peaks and the troughs of the sine wave of our consciousness.
We naturally love the peaks. During these positive phases, we find ourselves cheerful and purposeful. We feel cheerful because our attraction to Krishna gives us inner satisfaction. And we feel purposeful because our satisfaction affirms our conviction that we are on the right path and inspires us to press on enthusiastically.
Conversely, we naturally dread the troughs. During these negative phases, we find ourselves cheerless and purposeless. We feel cheerless because our apathy or aversion towards Krishna drains away all our inner satisfaction. And we feel purposeless because our inner dryness sows the seeds of doubts in our hearts, leaving us wondering whether we are on the right path and whether we might be better off by taking an about-turn.