As one of the Sages remarked, a person who takes pleasure in good food on a feast day is often enjoying his belly and not the holiness of the occasion.
Thus the feeling of joy in some outer spiritual circumstance will have its parallel in a physical source.
Because the physical sensation can be the same for both, there is a lot of room for imagination and error.
A person can imagine he is on some high plane of sanctity and actually just be puffed up with pride.
The sensation of spiritual achievement may very well be no more than an enhanced appreciation of one's ego.
To be sure, there are instances when the distinction is so grossly obvious that a person has to be cunningly able to deceive himself to get away with the fraud.
But there are also cases when the differences are very subtle indeed, and there are no objective standards to measure oneself by.
The purely subjective pleasure stands by itself in the midst of a question:
Where am I? What is the source of my joy?
–Rabbi Adin Steinsaltz
From “Elixir of Life and Death,” p. 217, from The Candle of God by Rabbi Adin Steinsaltz