Update #3 Oct 23: added comment
I know this post is old but i wanted to share this article (with lots more information on the website) on the “why”
https://www.soul-healer.com/energetic-healing/entity-attachments-releases/causes-origins-possession/
But ultimately (end of article + more detailed info linked at the end)
It makes no difference on which level Entity Attachments originate
Now, we don’t just have existences as a human We exist as many different forms in many different environments. Each of these places offers those parts and forms of yourself many different opportunities for living, expressing and exploring. Again you may make enemies, swear agreements, make love attachments, hate attachments and so on. All the relationships (with other beings) that we have within these places are a potential source of entity attachment. The higher the level the attachment is the more difficult it can be to deal with. On the other hand, these are in a general context very rarely a problem as the majority of humans are not multi-dimensionally open and often don’t activate these types of attachment. However as this capability is more prevalent at the present time then awareness of these attachments are becoming more common.
Also why 
shielding as a permanent solution is not a good idea…
https://www.soul-healer.com/psychic-energetic-protection/hidden-entities-healer-failures-awareness/
https://www.soul-healer.com/psychic-energetic-protection/depleted-spirit-energy-body-field/
And often the solution comes down to things like this
And grounding
https://forum.enlightenedstates.com/t/release-automated-grounding/42690