Gratitude

Rav Yaakov Horowitz z"l, RY Ofakim, related that his teachers, Rav Hutner ztz"l and Rav Shach ztz"l would distance people who didn't have a sense of gratitude [הכרת הטוב] even more than sinful people. The reason was that הכרת הטוב is the foundation of the building of one's world of good middos - יסוד בבנין הנפש היהודית.