Rabbi Elimelech ordered his Chasidim to refrain from wine and liquor during the latter part of Succot and especially on Simchat Torah—and this precisely because Simchat Torah is a special time for joy in the Torah.
According to Rabbi Elimelech, if the joy of Simchat Torah is mixed with half a bottle of vodka, there may be some difficulty in distinguishing it, because the sensations are similar.
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?
Let us take as an illustration the case of a man studying Torah.
He becomes elated at having found something new and interesting, a "chidush" (innovation).
And indeed the Torah may very well have revealed something marvelous to him and his joy may be a genuine intellectual elation unrelated to his ego.
Or it may be a feeling of exultation at having gotten the better of someone else, of showing himself to be more clever, more successful than others.
That is to say, it can be a joy of spiritual experience or it can be a joy of the shell.
This person can continue to study Torah and keep enjoying the occupation with Holy Scripture while all the time be involved in idolatrous worship of himself.
–Rabbi Adin Steinsaltz
From In the Beginning, p. 217, by Rabbi Adin Steinsaltz