Actually, there are two types of decisions people often (unintentionally?) confuse.
First is when one decides which path to take. This is a kind of decisions called “choice” – a decision based on estimations of benefits from each alternative. For example, one might consider buying a car or a buying bicycle instead, or even keeping riding metro because it’s cheaper and more convenient that way.
To make a proper decision in such a case one needs to account everything he knows about all the possible alternatives and their consequences. The choice with the most beneficial consequences will be the best choice. By failing to account every known consequence or constraining oneself to only a subset of known consequences, one may end up choosing the bad or even the worst.
Second kind of decisions is when one decides whether the assumption in question is true. This is called “proof”. These are the decisions based on the logic behind the things. For example, one might wonder whether the Earth really does orbit the Sun or whether his girlfriend really doesn’t cheat on him (incomparable things, I guess, but well…)
This kind of decisions due to it’s assymetry requires exactly one
argument “against” to shatter the hypothesis. On the other hand, no amount of positive arguments could bring that single undefeated negative argument down. While it’s there, the hypothesis is wrong, period. The one and the only possible countermeasure against negative arguments in this case is to disprove them, in which case they become effectively nonexistent and any doubts in the hypothesis vanish. Given that there’s at least one positive argument, in the absence of negative arguments hypothesis can be considered true.
In such a manner, to make a proper “proof” decision, one needs to suggest a logical explanation for a chosen alternative, and then, no matter how convincing the explanation might seem at it’s own, disprove any arising counterarguments.
Basically, when you’re dealing with a “choice”-type decision, you have a pair of scales. Every pro and contra you bring on goes to the appropriate scale, shifting the pointer. In the end, some of these contras might remain unresolved, but as long as they’re outweighted by pros, it’s all right.
On the contrary, with “proof”-type decisions you’re at a trigger gate. You need to be perfectly clear to pass. Weight does not matter; as long as you have even one discrepancy unresolved, it’s not good enough. A single contradiction is enough to sink your proof.
The thing is, people often confuse these two. They end up “proving” their statements by bringing up a shitload of pros (which does not matter as long as they ignore contras) in a hope that outweighting a contradiction would somehow diminish it. Or they do argue for their choice by stating that “it’s the only choice that allows us X” and completely ignoring the fact that it is also the choice that denies us Y, Z, A, B, C, et cetera.
Don’t do that.