Why is Layton Don Paolo's arch enemy in Professor Layton and the Curious Village?
The true answer to this question in Professor Layton and the Last Time Travel. Here it is. Professor Layton and a girl named... I believe it was "Claire"... were sort of going out and Don Paolo was IN LOVE with her. He got mad and became his enemy. And here is a small spoiler. That girl is the ONLY reason he wears his hat all the time...! You can look up to different videos on YouTube. A: Layton Takes His Hat Off and B: Professor Layton and the Last Time Travel Video 19/38.
Hope This Helped!
EDIT: In Professor Layton and The Unwound/Lost Future, Layton tells Luke that Claire was his girlfriend before but passed away in an explosion that was caused bu Dimitri Allen (Dr.Alain Stanhgun) and Bill Hawks. In a cut scene in the game, Claire gives Layton a letter and tells him in short that she loves him by giving him a kiss on the cheek. During That moment although they don't realize this, Don Paolo was EAVESDROPPING on the two and ran straight into the river saying" Layton!!! You will pay for this Someday!!! While Crying)