I like to think that come mai invites a longer, chattier answer than perché would. The last and biggest distinction between the two is that, while perché is both question and answer ('why' and 'because'), come mai is question only as you'd never use it to start an explanation. Don't miss any of our Italian words and expressions of the day: download our app (available on Apple and Android) and then select the Italian Word of the Day in your Notification options via the User button.