call me an asshole but instead of trusting or not trusting people, I trust that someone will always look out for themselves first and others second. I'm never surprised when they act that way and pleasantly surprised when they don't.
A lack of trust will also restrict your options. You'll spend more time making sure all contracts are watertight, or have to pass on opportunities if the legal system (which is the fallback option after 'trust') isn't capable to compensate.
It's also somewhat wrong on the facts: The idea of completely rational and selfish behaviour is called 'homo economicus', and it's among the worst of all the bad assumptions economics has made. Because as it turns out, it's almost impossible to find people acting in such a way outside of mental institutions.
The only rational and selfish test subjects are usually monkeys. Humans are willing to forgo all sorts of rewards for either altruistic or moral reasons.
The question is whether, in a team situation, one assumes that others are looking out for the shared interests as a group. If no shared interests exist, then the team has a much bigger problem.