Gabe Newell, long before making Team Fortress 2, loved the show Dora the Explorer. His favorite character was Swiper. He loved the cleverness and sneakiness of the fox, and eventually started recording Dora episodes on tape, and after copying the tape several times to make back-ups, he converted them to DVD and made a Swiper compilation showing every appearance of the fox. Unfortunately, Lord Gaben's Swiper compilation DVD suffered the same fate as my Inspector Gadget DVD.

When Gaben created the world of Teem Frothdress 2, he remembered his Swiper compilation. As a memorial, he incorporated Swiper's sneakiness and maskedness into the Spy, which became his favorite class. Swiper was incorporated into other classes as well, such as how the Scout is annoying but, most of all, the Sniper was most affected. The Sniper wears gloves like Swiper, and just like how Swiper steals Dora's crap, the Sniper steals ur kills in-game, hence the nickname "Swiper" which the Sniper earned for the same reason as Swiper from Dora.

A good example of the Sniper and Swiper's similarities is their posing.
Here is a side by side comparision of poses they both use:


Gaben knew that he couldn't incorporate all of Swiper's characteristics in the Sniper or else the Sniper wouldn't be a more generic class which would start ridiculous fights over the class uniqueness that would keep the game alive for centuries, so Gaben incorporated the rest of the characteristics in a couple other classes, for which I previously listed above, the Scout being annoying and the Spy being clever and sneaky. The Spy inherited the characteristics of Swiper that Gaben loved the most, which is why Gaben's favorite class is the Spy, however, the Sniper will always be the one who is based off of Swiper the most. This sacred ancient memorial is the basis of Teeth Friendfiest 2 in our technologically advanced modern day society.