Everyone likes the feeling of having success. And nothing is more depressing than a consistently growing bad feeling that something is not right or heading in the wrong direction. Typical example of such bad feelings: hearing or reading for the 5th time "it's 90% finished".
So what is the great mood enlarger in agile, iterative methods like Scrum? Simply its transparent progress of small and visible results!
And this is not only for product owners to quickly see what they expected but also for the development team to quickly get positive feedback and as well for scrum masters to quickly see that their mentorship is fruitful.
Quick satisfaction with your own work is a success key of being agile!