In the numerous discussions of personal opinion on the Vine, the question often becomes, How do you believe that?  This skepticism is warranted, as we are wired to expose deceit, just as we are wired to deceive.

Because there is deceit, there exists a strong selection to recognize when deception occurs. As a result, self-deception evolves so as to better hide the signs of deception from others. The presence of deception explains the existence of an innate ability to commit self-deception to hide the indications of deceptions. Humans deceive themselves in order to better deceive others and thus have an advantage over them. http://en.wikipedia.org/wiki/Self-deception

Lying is second nature to us and under the influence of self-delusion we'll hang ourselves if given enough rope...if we can only see our own point of view, we can authentically argue our case because our deceits blind us to the truth. Ignorance can be bliss, until you are outwitted by a perspective you don't share. http://www.guardian.co.uk/science/2011/oct/07/deceit-self-deception-robert-trivers

Though we profess to hate it, lying is common, useful and pretty much universal. It is one of the most durable threads in our social fabric and an important bulwark of our self-esteem. We start lying by the age of 4 and we do it at least several times a day, researchers have found. And we get better with practice. In, Like Lance Armstrong, we are all liars, experts say http://www.latimes.com/news/science/la-sci-lying-20130119,0,697761.story

What is the nature of belief, and in particular, self-deception?

...self-deception is the acquisition and maintenance of a belief (or, at least, the avowal of that belief) in the face of strong evidence to the contrary motivated by desires or emotions favoring the acquisition and retention of that belief.

Beyond this, philosophers divide over whether this action is intentional or not, whether self-deceivers recognize the belief being acquired is unwarranted on the available evidence, whether self-deceivers are morally responsible for their self-deception, and whether self-deception is morally problematic (and if it is in what ways and under what circumstances).

And yet insofar as self-deception represents an obstacle to self-knowledge, which has potentially serious moral implications, self-deception is more than an interesting philosophical puzzle. It is a problem of particular concern for moral development, since self-deception can make us strangers to ourselves and blind to our own moral failings. http://plato.stanford.edu/entries/self-deception/

Self-deception is a subset of false belief, intertwined with cognition and emotional factors.  It starts with an honestly held false belief, and then evolves as a justification for the belief, within the context of emotional coherence.

From Baljinder Sahdra and Paul Thagard:

...truth and desirability of a proposition become interdependent.

According to our model, self-deception occurs via emotional biasing of people’s beliefs, while people attempt to avoid or approach their subjective goals.

According to this theory, certain individuals tend to use self-deception in order to maintain their happiness. Such individuals are either positively disposed or they have high expectations of control.

...we hypothesize that there is a two-way causal-link between self-deception and subjective well being. There is evidence that self-deception causes subjective well being...Also, subjective well being causes self-deception...

...the way different causes lead to self-deception is through emotional coherence.

...the causal relations involved in self-deception [are resolved] by proposing that the mechanism of the causal relations is emotional coherence.

Thus, in wishful thinking, people believe everything that they want to believe. Self-deception, however, is a ‘weaker’ state in that we may successfully deceive ourselves into believing something, but not everything that we wish to believe.

...psychologists have shown that our judgments are colored by our motivations because the process of constructing justifications is biased by our goals. Nevertheless, we cannot believe whatever we want to believe.

As Kunda (1999, p. 224) puts it, “Even when we are motivated to arrive at a particular conclusion, we are also motivated to be rational and to construct a justification for our desired conclusion that would persuade a dispassionate observer.” There are constraints on what we can believe. In self-deception, we succeed in believing some (false) beliefs but not in believing everything we want to believe. Because some wishes remain unfulfilled, anxiety or internal conflict typically but not necessarily accompanies self-deception, but not wishful thinking.

Denial is also different from self-deception in that it is a kind of direct lie that self-deception is not. In denial, a person knowingly or consciously lies that ∼p. In self-deception, however, the person really believes that ∼ p.

Both claim something false, but in self-deception the correct belief (that p) is ‘held’ nonconsciously, whereas in denial, it is believed consciously. Also, in addition to denial, self-deception contains a very strong ego-enhancement component (Paulhus and Reid, 1991). Thus, self-deception and denial are importantly different.

And finally:

Emotional coherence occurs unconsciously. We are proposing that the unconscious mechanism behind self-deception is emotional coherence.

It is plausible to hypothesize that this tendency is due to emotions and that depending on the degree to which people are emotional, they may be less or more disposed to deceive themselves.

http://cogsci.uwaterloo.ca/Articles/self-deception.pdf

None of this may be particularly surprising to anyone, but these researchers provide a clear scientific explanation for the self-deception we commonly face here on the Vine, and in everyday life. 

Emotions, and emotional coherence are integral to belief and subjective well-being, which explains why logical proof and physical evidence seem to play a lessor role in these discussions involving personal opinion.