Why is Demi jealous of Ariana?

Demi Lovato has just about everything: an awesome voice, great style, a supportive boyfriend, the list goes on and on. But what's the one thing she really wants from Ariana Grande? Her song! "I wish Bang Bang had been mine," Demi reveals. Demi has even filled in for Ariana by performing the tune with Jessie J. "I loved singing that song because I felt like I was able to show off my vocals," D shares. "Jessie was so awesome about it. She was super sweet. We had fun performing it." We think all three girls should get together for a collaboration. How awesome would that be?