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Intuitively (dare I say logically), this concept appears to be a logical fallacy. Is it actually a "formal" fallacy and does it have a name?
Reincarnation must be true because I wouldn't want to live in a world where this is all there is (and anything goes etc.)
or
Theory X must be false because if it were true, well that would just be sad
In other words, conflating the essence of the idea with notions of its consequences that have no bearing on the veracity of the idea itself.
This fallacy is called wishful thinking. To give a more formal definition, the "fallacy files" defines it as follows:
I want P to be true.
Therefore, P is true.
Notice also the two exceptions where a valid form of reasoning can look like wishful thinking at the link. Along the same lines, if you are discussing a practical syllogism, it might matter that the conclusion would make you happy such as the following form:
If I were married to Sally then I would be happy, so I should marry Sally.
In such a case, the form is not fallacious because happiness is material to the conclusion of what you should do. Of course, it may be wishful to think that marrying Sally would produce that effect, but that's not quite the same thing as this fallacy -- that may just be folly.
