why I always thought of unpleasant moments in my life instead of pleasant ones?

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when I was lying in bed before falling asleep, I closed my eyes and remember of many things, but usually the bad ones made me thingking them again and again. For example,embarrassing experiences or,when I had done something wrong,how I feel sorry about it. These feelings remain clear and strong in my mind while the happy remoeries fade and finally disappear. I wonder if you have the same feelings or it's just my problem?