Attractiveness. Frankly, I think this is one of the most important reasons to adopt a positive attitude. It just makes you so much more socially attractive in all kinds of ways (with friends and random people you meet, at work/in school or with some cute guy/girl you’re interested in). Plus, a lot of people simply don’t have the patience, time or interest in a lot of negativity. They’ll just spend more time with positive people instead.
You focus on the good things in people. Not their flaws. This will make things much better overall and improve all kinds of relationships.
It’s easier to be more productive. You simply get more things done when you stop laying obstacles in the middle of the road in the form of negative thoughts.