When i'm sad I usually want to be left alone. I don't ask for help or to be comforted. At times like that I don't want to be bothered with anyone's else's problems, as I already have enough of my own. Not everyone is like this, and some people want to be comforted. The people they try to get help from usually don't want to be bothered with it either. When people are in a good mood, they don't want something sad to ruin it. Moni and Debiler have already said this.
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Everyone is entitled to their own opinion, I just don't have to listen.