Board Thread:Questions and Answers/@comment-44260486-20191028120243/@comment-31797469-20191203203826

As Pomelo said, Monster Level and Happiness does not affect breeding chance (despite in-game tips claiming so).

This has been confirmed by dedicated people manually testing using thousands of tries, at Monster Level 4 and 0% Happiness vs. Level 15/20 and 100% Happiness.