There are many theories about the best ways to teach and how learning takes place. There are literally hundreds of books and thousands of articles on the topic. But, I would put my money on one consistent finding. Students who feel safe and happy, and enjoy where they are, they tend to behave well and learn.
Simply handing out rewards to make students feel happy isn't what I'm taking about here. To be happy at school, students need to feel that they are undertaking work that is of value to them. This is different from happiness that comes from a teachers' approval or doing what is expected. Even elementary students understand the difference between a reward or recognition for doing what is expected and for success in doing their work.
Walking around school and seeing smiling and laughing students always brings a smile to my face, because I know that this is a place of happiness and that happiness leads to successful learning.