This assumption that I must think everyone who disagrees with me is delusional is something you bring up rather often. But each time you do, I tell you that I do not think that -- and yet you stick to it.
The problem is not that such people as believe in, say, dowsing, are delusional, but that their claims do not stand up to scrutiny. So why do they make these claims? Clearly such people genuinely believe that what they are seeing is real. That is not in itself a sign of delusion, it's just human nature. But, as I have said before and will say again, humans are just not that good at interpreting what they see in a lot of cases.
For example, there is a tendency to spot patterns where there are none, or to jump to conclusions on small evidence, as described here:
http://www.scientificamerican.com/article.cfm?id=patternicity-finding-meaningful-patterns
Meanwhile, human memory can be suggestible, mistaken or even implanted entirely (see, for example, Human Suggestibility: Advances in Theory, Research, and Application, Routledge, (New York), 1991., or Loftus, Elizabeth F; Pickrell, Jacqueline E (December 1995). "The Formation of False Memories". Psychiatric Annals 25 (12): 720–725)
Even if what has been seen has happened, such as people getting better after having taken a medicine that is known not to work, there is the risk of people finding causation when it was just coincidence,
cum hoc ergo propter hoc, which is probably the source of many ideas that can (at least for now) be called paranormal.
Yet more fallacies, each of which needs to be ruled out as the source for some claim, are very common and influence everybody to some extent. I've mentioned these before, but here's the comprehensive list:
http://en.wikipedia.org/wiki/List_of_biases_in_judgment_and_decision_making
Then of course there is the wonderful human capacity for imagination. It is clear in reading attempts to predict the future two things: that people's imaginations can turn out to be wonderfully accurate, and also that they can be completely wrong. Very often the same people can manage both at the same time:
http://io9.com/5873017/these-predictions-for-the-future-from-1900-are-eerily-accurate
All of this is just normal human experience, and should show that it's risky to base a theory solely on such experiences and accounts, without corroborating evidence that is free -- or at least, as free as possible -- from such biases and risks.