1. Medic is the best class to learn with. Even newer players can be immensely useful if they simply keep all of the players around them overhealed! Specifically, an overhealed soldier, scout, demo, or heavy can do much more then they can do without an overheal. By playing medic, you give your team a huge advantage, even if you never uber! Speaking of ubers, newer players usually have misconceptions about the charge. When using an uber, do not just focus on getting kills with it! An ubercharge is extremely important to save players from death by quickly switching it around, keeping territory by ubering against a push, and can even be used to save an otherwise lost cap/point. Protip: If you need to use an uber the second you get it, hold down right click to make sure it pops the second you get it.
2. Medic is a lot of fun to play with friends. There are few better feelings in tf2 then completely mowing over player after player with a trusted friend and a kritzkrieg.
3. Medic can be a lot of fun to play independently. While you should always keep people overhealed and build uber, going for ubersaw kills and crusader's crossbow snipes can make the class a lot of fun, while still being useful. However, do not use these weapons if the people around you are not overhealed. That feeling when ubersawing someone who thinks they are getting a free kill is great!
(some of the best medic frags I've ever seen)
4. Medic chaining! Simply put, a medic ubering another medic, him getting 4 ubersaw hits, then ubering the first medic, him getting 4 ubersaw hits, repeat until time runs out. Extremely high difficulty, extremely high trust required, extremely fun. What I think of as medic on hard mode. If anyone wants to, just let me know in game. Even more fun if you voice chat using steam with each other.
5. Appreciation. Everyone loves a good medic.
6. It's the best way to practice movement/dodging/surfing. Since you do not have to think about aiming, this allows you to focus on making sure you don't get hurt.
7. The ability to control everyone life. Someone harassing you in voice or chat? Someone take the hp pack while you burned to death? make sure not to heal them until they start behaving! Selectively healing accomplishes much more then raging!
8. Awesome hats
Anyway, hopefully we can see some more doctors on CSN. Just remember to have fun, feeling like you are force to play medic isn't the right mindset to play well.