Complex sentence: With their departure, Sonny is understandably depressed, but that quickly changes when Danny has one of the waitresses bring Sonny a drink because the waitress looks exactly like Kira.
Split into simpler sentences: With their departure, Sonny is understandably depressed. But that quickly changes when Danny has one of the waitresses bring Sonny a drink, because the waitress who brings him the drink looks exactly like Kira.