I have always had trouble working out why I can't make friends easily...I think the truth is probably that normal people can pick up on tiny abnormalities in our behaviour, abnormalities that we ourselves may not be aware of.
And the very fact that somebody has no friends, and walks around on their own, eats lunch on their own, ect, can be seen as very strange in itself.
I have found that I can talk to people effortlessly over the internet, and I have met many people in this way, although I wouldn't consider most of them true friends.
I recently moved into a kind of residential therapy centre, for people who suffer from mental illness. Here I have managed to make a real friend, who has problems herself, and is quite understanding towards me.
My technique for making friends is to find someone who shares an interest with me, such as computer games, or animals, and talk to them. Once we start talking, I have found that I tend to "parrot" their behaviour. To put it simply, if they are affectionate towards me, I match their affection. If they joke around with me, I joke back. I'm not sure if this is the right way to do it, or if it is something that everyone does, but it seems to have some effect.
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Effect of Blood Plasma from Psychotic Patients upon Performance of Trained Rats