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A positive attitude doesn't just mean thinking good thoughts. It means not getting caught up in the seriousness of everything we do, hear, see, feel, and relate with. ...it doesn't help to give up on the things that we're serious about. That doesn't really serve the purpose. What helps is having more lightness and humor.
When you find yourself stuck in seriousness--even if it's your karma to be a very serious person--you can just pop out of it. This is quite a profound practice. --Dzigar Kongtrul Rinpoche