When is a formal letter inappropriate?

You would not want to write a formal letter for casual writing to someone you are close to. For example, if you were writing to tell your grandmother you would like to see her over the summer, formal would be out of place.

Dear Grandmother,

I am writing to inform you that you are missed. I would like to schedule an appointment for summer visitation at your soonest convenience. Please contact your daughter, Jane, to verify your times. I look forward to your anticipated cooperation.

Sincerely,

Little Johhny

A casual letter would be much more appropriate.

Hey Grandma,

How are you? I miss you so much! I really want to see you this summer. When you have a chance, would you please call mom and see what days I can come over? I am excited about seeing you!

Love ya lots,

Little Johnny