Hang out with likeminded people and it'll become contagious.
Case in point. If you hang out with someone with negative attitude long enough, you may just start developing a negative attitude.
If you hang out with someone who is positive, you may just start to develop a positive attitude.
It sounds hard, but it really is simple. One of my favorite quotes goes; "if you hang out with dogs, youll get fleas".
*just a little food for thought*