How do we decide what we decide? For me that question used to have a simple answer. We assess the evidence and then come up with the determination that best fits the data. If people disagree, it's because of one's lack of understanding or misinterpretation of the evidence.
Yet, we have many very equally well-informed and intelligent people who disagree on topics, people who know the facts and make a sincere, persistent effort to look at those facts and come to a rational conclusion. They have pretty much the same facts and are intellectually honest, yet they come to different conclusions. A + B should equal C, but they end up equaling D and E. Facts are facts, so the variance doesn't lie with them. Rather, it lies with the other element - the human thinker.
Some might argue that the different conclusion lies in some basic fault in one of the individuals. They would challenge my basic premise that the two individuals are "equally" well-informed and intelligent. One of them must be missing a piece of information or not seeing an important element. Or, one of the individuals must not be well-intentioned; she must not be trying to truly understand the facts or must have some underlying bias leading to misinterpretation. Perhaps. But the default often seems to be to point to the individual's human shortcomings as a way to dismiss the argument, the ad hominem strategy.
In the context of interpersonal relationships, this strategy can be a dangerous one as it can demonstrate a level of contempt for the other person preventing any kind of productive discussion. "I don't have to take into consideration what the other is asking or presenting, because they don't deserve my consideration; they are beneath me, and any conversation is pointless, because the other is incapable." One need not truly consider another's perspective since that other person lacks something basic to legitimate discussion i.e. intelligence, knowledge, basic decency.
So, what to do? Being content with the possibility that one could be mistaken about a tightly held belief seems to be an important aspect.