When you are experiencing an emotion, your mind effectively filters out any information that doesn't reinforce the emotion. This is why it's very hard to make a make someone joyful suddenly feel sad, and vice-versa. This is perfectly normal and healthy. The problem is that we also experience moods, these last longer than emotions and basically give you a predisposition to feeling a certain way. So a depressed mood makes it very easy for you to feel sad, a cheerful mood makes it easy to feel happy about thing. Again, this is perfectly normal and healthy (though arguably not as beneficial as just experiencing emotions).
People who are really depressed are effectively in a depressed mood almost all of the time, they may occasionally have good days (this can be a result of being too busy to actually be emotional, or only having positive things happen, or it may be one of those rare reprieves) but ultimately they are predisposed to experiencing depression-related emotions and it's extremely difficult to see the positives (because of the way emotional filtering works).
So, if someone is experiencing real depression, there is almost nothing you can say or do to snap them out of it. I'm not going to argue the causes or treatments, but if you want to help a depressed person you're going to have to put up a lot of grief and little reward for quite a while until you basically become a rock for that person. If all you have to say is 'suck it up' or 'grow a pair', or even 'cheer up', you're not helping.
Detty. Professional Expert.
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