Surprise parties can be incredibly hit or miss. When my mom turned 12, her friends and family threw her a surprise birthday party. She got so startled by the surprise element of the party that she cried for 30 minutes. Luckily, not all surprise birthdays are such a bust. When I turned 16, I had a fantastic surprise birthday party that ended with my friends comedically roasting me. Whether they go wonderfully or terribly, surprise birthday parties work because birthdays aren't that novel. If you have a terrible surprise party one year, it's not like your birthday is a once-in-a-lifetime phenomenon. You can have a 13th birthday party the following year with no surprises involved.
Certain parties must not be surprising, and weddings are one of them. A woman recently went viral for sharing her story of her boyfriend (not fiancée) throwing her a surprise wedding, and the internet is lit up about it.