Maybe because he did not explain it simply and purely. You do not "thank" people who are close to you; helping and favors should simply be a common practice.

Maybe because he did not explain it simply and purely. You do not "thank" people who are close to you; helping and favors should simply be a common practice. "Thank you" is said to people who you cannot return the favor to, or do not expect them from them any sort of kindness.