I find this to be true for Spanish speakers too. It varies from person to person, and how their family is. For example, my Mom uses informal speech with her family, but my Dad uses formal with his ("Usted"). Thanks for your help! :)

I am using less informal words to my mom and a bit formal words to my dad. :) Now i come to think of it and it's not fair though. 왜 그러는 지는 모르겠어요. Guess I feel more intimate to my mom. And it's quite common to other families too.

감사합니다

Do the formal/informal speech mean 반말/존댓말? If then, it is due to the parents. I have spoken to my parents using 반말 since I was young. But some of my friends are using 존댓말, and other friends are mixing 반말 and 존댓말. It is decided from the mood of their family.