"It takes many good deeds to build a good reputation, and only one bad deed to ruin it"

I wonder why this is..

Trust is hard to build because we are all mortal and human life is so fragile. If you live in an untrustworthy world, it is very easy to be killed. Two people standing next to each other can very easily do mortal harm to each other with only their hands.

A person who acts suspicious makes us wonder if they are following the same moral code as we are and how they could harm you, even inadvertently without any harmful intent.

Part of me thinks that it is because our default is to not trust people, and it is always a matter of work to move away from that default towards a relationship with trust.

But then I like to think that most people have a fairly high level of trust to begin with, so it is probably more of a reaction to getting hurt or taken advantage of when it happens.

Having a high level of trust in other people is where you need to reflect on the society you grew up in. There are many places in history and the current world where life is cheap and people will spend human lives for their own gain. In those times and places, I would wager many people would not have a fairly high level of trust when meeting a stranger.

If you have high levels of trust in strangers, this is not a bad thing, but rather a reflection of the era and place that you grew up in. It's likely that your ability to trust was hard earned by history and institutions you trust in.