You need reasons to believe something, not just a lack of evidence to the contrary.
The answer stands: No. It is not the main reasoning behind our belief in anything. It's not how the human mind works, generally speaking.
Exactly, people always need a reason to believe something. We agree about that. But that reason is not always based on evidence or the lack of it, and often the main reason why someone has chosen to believe something can be stronger than all the evidence in the world that what he believes is not true.
Eg religious sentiments and the influence of religious communities on a person can be much stronger than evidence. When a particular person is a respected member of a religious community that preaches creationism, he/she can decide to stay a creationist even after seen irrefutable evidence for evolution. Because if he/she would accept that evidence and change his believe about creationism, he/she will be in deep trouble with his/her religious community and lose respect, friends and family. If for this person his position within his community is more important than the truth about evolution, he will stay a creationist no matter what.
That's how the human mind works, generally speaking, and the driving force behind self-deception.
Therefore as soon as you see someone denying irrefutable evidence when that irrefutable evidence doesn't support his belief, (like the declining speed of the terminator on the southern hemisphere) you should start questioning the reasons of his belief; a desire to know the truth........ or something else.