Stubborn people problem! Can you help?

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I have a problem with my stubborn friend. He is quite a negative kind of a person in constant depression. I've tried too cheer him up, talk with him. I even tried to encourage him to change something in his life. I know him for few years and he really doesn't have any such problem to be like this although he is very stubborn. Making him do something is almost impossible, couse he always thinks that he knows better. Do you have any ideas how to convice a stubborn person to actually do something that he don't want but must do?