Depends on what you consider thinking positive, I always liked the joke that an pessimist is an optimist with enough life experience, but that doesn’t apply to every situation. First of all, you cannot expect the positive without accounting for the negative (in basically everything situation from stock trading to cancer), plus you always have to expect that other people will act in ways that are bad for you (e.g. if you buy something used on ebay), so a good portion of doubt is important, but don’t always expect the worst possible outcome.
A simple example for this is any kind of con operation, this is based on the fact that people are naive enough to expect a good outcome but not account for the bad possibilities, e.g. if the widow of an African dictator sends you can email that they want to transfer you millions of $, you cannot expect to get anything, even though expecting the positive would mean that you might get some money.