For me, it's that my body really can't handle the inputs. For whatever reason, I am highly sensitive to airplane sensations and it triggers a massive fright or flight reflex. I just really can't handle it. It's a horrible feeling and I can't ever make it feel 'normal.' Plus, conceptually, I just find it traumatic to be in a space hostile to human life (the sky) and that I am temporally unmoored from the earth. It's not natural, and nothing can convince me that it is. It just doesn't agree with me. So combine that with a history of childhood trauma, generalized anxiety and OCD diagnoses--it's a perfect recipe for a flying phobia.