From my perspective, this (and all other conspiracy theories out there) all come down to three things: truth, trust and belief.

Truth: I was not there. I did not witness it happen. Even if I did witness it, I would have had to be on the plane to see what happened there, on the ground to watch the impact or in the building watching the approach. Fortunately, I was not. I will never really know the complete truth from any of those perspectives because I was not there.

Trust: Because I was not there, I must take into account other sources of information about what happened. I have to listen to witnesses, read reports, view media. But I must gather this (potentially conflicting) information and form judgements. What do I trust? Does this make sense? Much of the blind faith that was formerly put into both governments and media has been replaced today by distrust. Just or unjust, trust needs to be earned and is easy to break.

Belief: Eventually, I will formulate my own personal understanding of what happened. This is as a result of filtering the data from the "trust" phase and connecting what I know to be "truth" from my own perspective. This result can and does change over time based on changes in the underlying facts. Because this is a unique process for each individual, one person's belief will be another's fantasy.