1. There's so much to do.
Living in a small town can (is) boring. But in a big city, there are endless activities like grabbing a bite to eat, getting drinks with your besties, going by the beach or lake, joining a gym, browsing shops, visiting museums and attending concerts. There are many options and being bored is not one.
2. Self-growth
Living in a big city pushes you to new limits you never knew existed. It can be painful to be pushed out of your comfort zone, but you know what's more painful? The regret of staying in a place that doesn't push you to be better, every day.
3. Meet all kinds of different people.
It is undeniable that you meet all different types of people. There are so many people in this world. Sometimes people are naive to diversity because of how isolated they become living in the same place all of their life. Meeting various types of people will expand your horizons.
4. You can be yourself.
Living in a large city, means it's you versus the city. You don't belong to anyone else -- you are solo, an individual, independent, on your own... you get the point. People won't base their perception of you off of the social circle or groups you are in. There is a sense of freedom in knowing that you can walk down the street and see that no one that knows you. And that is a glorifying feeling.
5. Career possibilities
Getting a job isn't necessarily easy, but there are a lot more job offerings. You don't have to feel stuck with a job that you hate because there are always alternative choices. Most large corporations are based in big cities, so that dream job will be closer to reach. Building connections with employers is more common in large cities too.
8. Magical
No one can deny the unexplainable magic of living in a big city. The traffic, the tall buildings, the lights, the constant noise... It makes you feel energized, all hours of the day and night, almost like sleep isn't necessary. You can walk outside in the middle of the night, but the city won't be sleeping; people are out all the time. The city never sleeps.