There is no explanation as to why videos go viral. They are either funny, dramatic, unbelievable or all of the above. What goes viral and what doesn't cannot be predicted. In fact, you can "make" viral videos. It has to happen naturally. 

Take this video on Mashable, for example. It has to do with a little kid, a dog and poop. Yes, you read that right. Poop.

Then check the speed at which the video caught the attention of readers.