1. HTML 5
With HTML 5 a whole new kind of web applications is possible. You are now able to develop full featured apps that run on any kind of devices - from mobiles to desktops.
2. Create stuff
Being a web developer gives you the power to create new cool things. If you can imagine it you can build it (or kind of). You don't need any kind of material - just your knowledge about web development.
3. Low Starting Costs
Starting with web development costs very little. Assuming you already have a computer and an internet connection, you only need some webspace when you website is ready for launch - and even this can be free.
4. Share it with the world
Websites can be reached from all over the world. If you think about, you realize how powerful this is. You can literally reach every person on this planet.
5. Job market
The web is still growing and web developers are a scarce resource that have good chances on the job market.
6. Mobile Devices
With mobile device getting more powerful, there are even possibilities for web developers to create cool stuff
7. Fast Results
Unlike traditional desktop programming, web development gives you instant results. No more dump "Hello world!" programs anymore.
8. It's easy.
Beginning web development is relatively easy - you only need your web browser and an editor. Both should be installed on your computer already. Don't get me wrong, it takes a lot to develop great web applications like e.g. Google Docs, but barrier of entry is very low.
9. Being creative
Web development let's you express yourself. You can try everything you want and share it with the world.
10. It's Fun
Most important, web development is a fun experience.