Why do we presume to be stronger or more complete? What if their energy is like an invisible virus penetrating human minds for good or bad purposes?
We presume---because we are a race that is by nature egotistical, and narcissistic. They are in charge! If you doubt what I say, watch "Ghost Hunters." Ghosts do not perform on demand. They communicate with you because they choose to.
I doubt seriously they have the ability to plant anything in our minds like a virus. I have however, experienced their lives and on occasion their death, but this communication was their choice and because they needed to tell it. There were never residual effects----except perhaps a feeling of profound sadness.
I don't believe I am in control of any spirit when speaking with them. I don't believe they are controlling me either. I believe we are interacting because we both chose to, just as you and I are interacting due to a common choice. I don't believe they are infecting us when we interact with them. However I do believe possession does happen and will happen when there is a spirit with the intent to do so in convergence with a person who is either weak or untrained regarding how to keep themselves protected.