Be all for something/for doing something
Definition 1 of 1
Pro Tip 1/3Enthusiastic Support
Use the phrase when showing strong, eager agreement or excitement towards an idea. 
I'm all for your idea to start a book club.
Pro Tip 2/3Positive Context
The phrase is often used when the speaker perceives the situation or proposal as beneficial or positive. 
She was all for moving to a bigger office space.
Pro Tip 3/3Emphasis on 'All'
Emphasize the word 'all' to show total agreement; it suggests no hesitation. 
They are all for adopting the new policy.