Google Flights shows some custom messages after you select your favorite flights. Depending on your destination, Google shows messages like "London, baby!", "Ahh, Paris... bon voyage!", "Have a great time in the Eternal City!" (Rome), "Have a great time in the Emerald Isle!" (Dublin), "Enjoy your trip to the Windy City!" (Chicago), "Have a great time in Music City, USA!" (Nashville), "Have a great time in Baltimore, hon!".
For most destinations, Google shows generic messages like "Hope you have fun in Frankfurt!".