It's important to 'celebrate' a bit once you've reached a goal to boost your confidence and morale, but dwelling on the accomplishment leaves you complacent with less motivation to improve. In management, it's known that never congratulating your team on victories leads to low morale and a feeling of hopelessness (things are never good enough and they never will be).

So as you mentioned, it's a balance to appreciate both sides in a healthy manner. There is also an extreme of developing a personality that is never satisfied and can reach very high levels of accomplishment though never feel happy. From the outside they seem very successful, but internally they are miserable.