Sharing Conversations & My Good Friends

I'm learning about sharing but I'm confused why daddy & mommy always tell me to share & then take things away. Granted usually they are things like scissors & permanent markers, but why not share everything? Below is an example of some of the sharing conversations going on around here. And yes, this is an actual conversation occurring merely hours ago.

Me: "Thank you for being a good sharer mommy."

Mommy: "Oh yeah...What did I share?"

Me: "Your drink."

Mommy: "Oh, I didn't know I shared my drink with you."

Also today I told mommy that we're good friends & that daddy is also a good friend of mine. If you can't be good friends with the people you play with the most, then you need to change your attitude I say!