This is simply speculation, but I think primals are nothing but huge amount of aether given corporeal form, just like crystals. Crystals of aether are known to cause dizziness and sickness from direct contact or near vicinity, so I think it's similar to primals, if you get exposed to primals aether too much, you'll get something like drug effect; you get addicted to it and your mind broken, only wanting for more aether. I think this is why some companions survived from becoming a primal's thrall; they never get exposed too long to be addicted, and the primals they encountered never showered them with huge amount of their aether.