The cause of the false identification of the Seer with the seen is due to ignorance. Ignorance in this context means lack of spiritual understanding. Avidya or Ignorance is at the root of the Kleshas or afflictions and the source of our suffering.
The body goes through changes—from baby, to toddler/infant, to child, to adolescent, to adult, to elderly but one thing remains constant the one who witnesses all within, the one who witnesses the changes that takes place on the exterior.
Likewise the mind is in a constant state of flux—constant change of internal images, fluctuation of moods, thoughts ebbing and rising. However only one aspect remains constant, the one who witnesses the flux.
The body is coalesced energy organized into an integrated system making it appear a solid unit. The mind is the subtle energy infusing the body unit as awareness.
At death the body disintegrates into its constituent elements: ‘ashes to ashes, dust to dust’.
At death the mind with all its desires and karmic propensities (samskaric bank account) finds another suitable vehicle (body unit) to express itself.
The Witness / The Seer being the witnessing agent of Consciousness remains unchanged purely witnessing the Show, the drama, the comedy of it all. In essence it never takes birth and therefore never experiences death, only a transition of form to continue witnessing existence which is none other than Consciousness in expression.
Not understanding this is what is termed living in ignorance.