We all have limited experience and knowledge, but this doesn't keep us from having to make decisions about things we are personally next to clueless about, or powerless to directly affect. For such decisions, we often depend on others we trust, who appear to have relevant knowledge, abilities, and experience that we don't. That trust tends to be based on our assessment of how much those others would agree with us on things we do know about. Conversely, the more different someone is from us, the less likely we are to trust them to do anything, especially if the potential cost of letting them is especially high. This dynamic applies to a wide range of situations, including who we vote for (in a representative democracy), what we buy at a store, and who is guilty in a court trial.
Trust also depends on our shared values. In my personal value system, a “good” decision is one that maximizes satisfaction and health for everyone who will be affected by the actions taken in its aftermath, with priority placed on the latter. Some would argue that it is they who should experience maximum satisfaction, and it doesn't matter what happens to anyone else as a result (they might even consider the diminishing of other people's health as a source of satisfaction). Others might quibble that “satisfaction” includes “health”; I've included both to ensure that health is taken into account regardless of the immediate goals of the decision.
As our experience builds with those we trust, we will either gain or lose confidence in them, and will either limit or expand their role in future decisions. Sometimes, however, we have no choice, such as when children must trust their parents or guardians to keep them safe. In such cases, the trust is often backed up by a society that imposes strict penalties for abusing it.
Our societies and their institutions are also held to standards of trust, by the citizens who are part of them, and dependent on them for what they can't do themselves. For example, the economic crisis that started in 2008 was precipitated by the abuse of trust by banks and their insurers. The U.S. government – as an agent of society – was entrusted with keeping them honest and their customers safe from fraud; yet that trust too was abused, and to date neither the institutions nor the government have taken adequate steps to ensure that they can be trusted about such things in the future (though people still depend on them, largely because they have little choice).
This brings up the question: What can people do if they have no one to trust when they are lacking what it takes to meet their needs on their own? Under such conditions, they may begin to form new associations, or even new communities, that are more trustworthy. The alternatives may even coexist with the original, untrustworthy ones until either trust is restored in the originals, or the originals lose all their power because no one trusts them any more. If no alternatives exist or can be constructed, then tragedy may result: slowly, if the originals still have some capability to meet people's needs; or quickly, if they have already fallen (the political variant of the latter is anarchy, where it's literally “every man for himself”).