In Diary of Anne Frank (play), Why is Mr. Frank grateful to Mr. Van Daan? What does Mr. Frank's actions toward the Van Daan reveal about him?(Glencoe Edition)

Mr. Van Daan helped Mr. Frank find a job and run his business when the Franks first moved to Amersterdam from Germany. Mr. Frank's actions toward the Van Daans clearly shows that he feels as if he owes them something and wants to repay their kindness.