It's kind of obvious...
Either for fun, dare, or just to see how long they could last. Have you never done stupid and fun stuff like that when you were a kid lol? Well... not that stupid maybe but...
when I was a kid yeah, but not as an adult.. Like these two people were.
From the looks of it they were like "OMG LET'S GO" then when it started getting intense, they were like
"LUL NEVARMIND LUL I RUN AWAI NAOW I LETZ GO TUU"