If someone is beside himself, he is extremely distraught.
You might even say he is out of his mind, because the ancients believed that under extreme distress the soul left a man’s body and stood beside the human form, which left the subject literally “beside himself.”
This absence of the soul gave the Devil an opportunity to fill the void.
Extreme pleasure could also cause this condition.
The Greek word ecstasy means “to stand out of the body.”