Sounds like the work of Joe Jonas. They did just spend time in Walt Disney World together walking around Epcot, taking pictures with Mickey Mouse, and singing their duet "Make A Wave" at a private concert. Plus, after their quick vacation ended, Joe tweeted, "@ddlovato your awesome! I love u bff!"
But alas, Joe is not the cupid culprit...
Frankie Muniz is. Seriously. He replied to Demi's tweet with a simple "you're welcome" and she replied "hahahaha I KNEW it was you... ;)"
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