We do things because we love to win.
But we learn more from failure than we do from success.
Whilst achieving success can often be the result is learning, once success has been achieved there can be a tendency to be complacent: To simply repeat the formula that achieved that success, without any ammendment. When we do learn in the course of achieving success the things we learn tend to be fairly minor and often, through perhaps some sort of arrogance, we can turn a blind eye to important lessons, believing what we do to be perfect.
Someone who is failing, on the other hand, often stands to learn a lot. They can learn by observing the victors and immitating their success. Because they want to win they are willing to learn new approaches and, in some cases, try radical techniques in order to achieve success. They don't turn a blind eye to advice; they positively welcome it.
The problem is that people can be turned off by failure. Failure can be dispiriting and, if someone does not believe they can succeed, can result in someone not attempting something, in which case they will learn nothing at all.
Teachers know that success is important to students for the above reason. But have we gone too far? Have we forgetten the importance of failure?