The previous Aethyr, ARN, is the only Aethyr that you can safely join with your divine self and still maintain your existence within time and space. In join, we mean unite completely with that divine self. In the lower Aethyrs, this union was an increasing graduation of the union accomplished in ARN and here in LIL. However, in LIL, the first Aethyr, the union of your beast and your divine self will result in the annihilation of all lower forms of your being. You will cease to exist within the physical development system and move past the final Ring-Pass-Not into Divinity, of which little can be said.
Because of this fact, it is very difficult to unite with your polar opposite when you are here. This is a safeguard to prevent you from annihilating all forms of your human self until you are ultimately ready. This failure to unite with your divine self does not mean that you can not still experience the love, completeness, and power from your union in ARN. It merely means that you are prevented from taking the final step to leaving the physical system until ALL of your human qualities have vanished.
In ARN you are a divine being that is human in its nature. In LIL you are a soul shedding the last of its human qualities to become divine.
* additional source material: "Enochian Magick" by Gerald J. Schueler, "The Golden Dawn" by Israel Regardie