Depending on the experience of the person dealing with stress it can be motivating for most people. Personality types have something to do with how we react to stress. Obviously an introvert often is not to appreciative of stress in their life. In Daniel Pink's new book "Drive" he refers to motivation from standpoints of survival, reward and punishment or your own intrinsic drive. Our most rewarding motivation and stress is with our intrinsic self-motivated push. You are correct when you see stress in different situations may be welcomed or not. But in any case mentioned above it is our own inner reactions that cause our stress.