only listen to your friends problems if you are in a good position to help them (i.e you have been there before) otherwise they may start to see you as just someone to talk to. think about the things you like doing, and see if you feel like joining any clubs to meet new people.
i have similar problems myself often, and i also suffer from depression, and i understand that you dont always feel like going out and smiling, but sometimes listening to everyone elses problems can only weigh you down.
i know a particularly bitchy girl in my school talks about me behind my back as the "fucking manic depressive", which i dont find amusing at all, because she clearly doesnt understant depression.
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dont wonder why im not smiling, wonder why you are.