This is because they are better able to cope when life gets more difficult.
This experiment focused on people who joined online groups. Those who were strong willed scored their life as happier than those who were not.
People who are strong willed also score higher in exams than those who are not.
Bernecker, K.; Herrmann, M.; Brandstätter, V.; and Job, V. (2015). Implicit theories about willpower predict subjective well-being Journal of Personality DOI: 10.1111/jopy.12225