Holelus means letting go by becoming less than what one really is. The person forgets about himself and about anything that has meaning, content and purpose. In the extreme, this means becoming drunk, or taking drugs that rob one of control. But it has far more common expressions. A person thinks that the only way he can be happy is by forgetting about everything but the sensory pleasure he is receiving at the time. He lives for the moment.
This can be very destructive, for when a person ignores responsibility, he is likely to hurt himself, his family and the people around him.
Simchah, joy, also involves letting go, but it is a very different type of letting go. One does not lose control – one transfers control. When a person experiences true joy, he lets go of himself, but he connects to something higher, G-d. He lets go of his petty ego and makes it possible for a dimension of his identity that is far deeper and far truer to surface.
This is one of the reasons simchah is considered a high level of Divine service. For this selfless connection with G-d – over and above all the advantages one gains by avoiding depression – is a goal for which we should all strive.
(From Rabbi Shloma Majeski’s The Chassidic Approach To Joy)