Based on Bhagavad Gita Chapter 02, Text 44
All material pleasures are essentially chemical highs. Whenever we perceive any attractive objects with our senses, the corresponding sensory mechanisms secrete certain chemicals. This chemical secretion gives us a temporary titillation that we call enjoyment. As our bodily capacity to secrete any chemical is limited, the enjoyment that we can ever get from any chemical high is also limited.
On the other hand, spiritual happiness exists in a realm far beyond chemical secretions; it depends on the connection of love between the soul and Krishna. This connection, being spiritual and eternal, floods us with a happiness that is free from the limits of matter and time.
We may wonder, are spiritual joys too not chemical phenomena? No, because their locus is in the soul. Of course, as long as we are embodied, all our emotions will generally correspond with a chemical expression in the body. However, spiritual emotions, unlike material emotions, are not limited by the secretory capacity of the body and can continue even when that capacity is exhausted.