Robert A. Monroe wrote in his 2nd book "Far journeys" that a friend of his suggested the reason for why he had OBE.

The reason was seemingly surprising: the roof of the house in which he lived resembled a pyramid. The reason for the OBE is energy.

I'm actually certain that it is true: the difference between being able and not being able to astrally or ethereally project is energy - how much you can gain it. Ancient Egyptians called this energetic part of self "Ka", taoists used the direct term "Qi" for energy in general. There are many techniques how to increase level of energy of Ka / Qi part.