Why is YouTube so popular?

YouTube hosts the largest number of videos available in a single place on the internet. This makes it the number one source for video material for many people. 

YouTube is owned by Google who regularly promotes it in its search engine results. When a keyword is searched on Google, it automatically suggests any related videos as well.

People like listening to music and YouTube hosts just about every musical video ever published. Anytime people want to listen to a song they can find it on YouTube.

YouTube hosts many funny videos. People who are looking for a quick laugh can easily find such videos on YouTube. (See Why Talking Tom Cat got so popular?)

Sometimes users upload entire movies on YouTube. Anyone looking for an easily accessible movie can perhaps find it on YouTube.

YouTube was the first major video website with user generated content and thus attracted its audience first among all the similar websites available today. (See How Did YouTube Start?)

YouTube is easily accessible from a computer, TV, tablet or smartphone. Its 'ease of use' approach makes it accessible to people of all sorts of technical competence.

YouTube focuses on user generated content, unlike TV. Anybody can upload a video on YouTube and make it available to the entire world.

Many celebrities such as musicians have official YouTube channels and have fans following their channels.

YouTube has rare and pretty hard to find footage available. This fact makes it a popular source when people are looking for a rare or historical footage.

YouTube's interface is translated to many languages. This makes it easier to use for those who don't understand English.

Most movies and games have their trailers released on YouTube first. Months and months of media hype culminate on a YouTube page with millions of people watching the trailer.

Back when YouTube first launched its website, it wasn't very affordable to have a server capable of holding a large number of videos. YouTube servers made it easier for anyone with video content to get wired to the internet.

Most people like to procrastinate and waste time instead of facing their life problems. YouTube is a great tool for diverting somebody's attention to less serious themes.

Many interesting, high production documentaries are hosted on YouTube. In addition, many filmed lectures can also be found there, providing an excellent opportunity for education.

Many exclusive shows are filmed just for the YouTube audience. The popularity of such shows boosted YouTube's popularity.

YouTube made it very easy to upload a video and release it to the public. Before YouTube, this was an expensive and non-trivial task.