Good question. The trick is not entirely trusting anyone. You should always keep in mind that people make mistakes, and that could potentially be harmful to you. So, just take proper precautions and adjust your expectations of people. Protect yourself.
sancarn
Do you want the evolutionary answer to why we trust or the short "romantic answer" of "we trust because we love and we love because we're human!" or some shit like that xD
inspiration1
because 'trust' is the most important reason/ factor for happiness.