If one is "overly optimistic" all of the time, then they are not someone who should be asked for or listened to for advice because they are not ever objective. Don't get me wrong, it's great to be optimistic, but it can have negative consequences too. People get killed playing Russian Roulette.
When making a decision, the question is not "will this decision make me happy" because "happy" is in the moment and is fleeting. The question is, "Will this enlarge or diminish me?" Those are long term effects that I consider when making an important decision. "Happy" is often the easy way out because we all want to feel "happy".