Positive thinking is very impotant in life and also an attitude that is required in business life. There is a dfference of being cautious or living with risks.
If being pessimistic prevents decisions to make progress then progress cannot happen at all. But if an optimist takes the risk then progress could happen, even if their would be the risk to fail.
And in in our daily life it is not so different. Just think if you would be a pessimist and never would talk to the women or men you are interested in, you would never find out what could have been.
An optimist may head straight forward, sure he also may does not get what he wants, but a least there is a chance.