Assuming good intentions is truly the only way to build relationships and stay sane.
Some people might act out of malicious intents, trying to manipulate others in order to get ahead. Most though go about their lives in the exact same way as we do. They might have different ideas, points of view, preferences. But they are trying to do their best with the cards they have been dealt.
The alternative is to question everything and doubt everyone.
It is possible, and some choose that. I am not sure though they understand how much energy, attention, resources that is taking from them.