Why can't I show affection to my family?

I have no issues telling my friends I love them. I do it all the time and show my appreciation for them and hug them etc etc. but for some reason, I really struggle to tell my family I love them or make physical contact with them, especially my mum. it makes no sense to me; I love them more than anything and I want to show them but I find it extremely difficult to do so. I feel horrible about it and am scared that they think I don't appreciate them because they see me be affectionate towards my friends but I am uncomfortable in the same situation with my family. is it normal? what do I do?
Why can't I show affection to my family?
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