One of the things psychologists used to say was that if you are depressed, anxious or angry, you couldn't be happy. Those were at opposite ends of a continuum. I believe that you can be suffering or have a mental illness and be happy - just not in the same moment that you're sad.
Martin SeligmanRelated topics
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I saw a bunch of the weirdest, oddest people I have ever met in my life, who thought different, and acted different, and even made love different. And they made me laugh, and get angry, and be happy, and be sad, and excited, and even fall in love a little....And they didn't seem to be so weird or strange anymore.
Samuel R. Delany