Yeah, seriously. I'll get to you. I'm talking.
There's many of reasons why people become a medic in a first-person shooter. They're especially true in TF2.
For me, there's two big ones. I'm sure there's some people who do the healing thing out of pure altruism. Hell, there may even be pacifist players out there, who doesn't believe in ultra-violence even when it's as slapstick as a paint-balloon. There's certainly some who enjoy being that go-getting team player - not true altruism, but the pleasure of being a cog in a well-oiled machine. In the case of Team Fortress 2, someone will be attracted to the clinically-dry persona of the Teutonic Medic, half-way between a Nazi Evil ( not mad) Scientist and the voice in a GPS guidance system. You may even just pick it by accident. But not me. Here's why I play the medic more than anything else...
1) I don't need to shoot straight.
2) I like to win.
Let me explain.
First one is obvious. My skills are weak. Or - more accurately - not as strong as I'd like them to be. More importantly, in these early days of TF2, I - and everyone else - is still feeling out the maps and learning what's up. As a Medic, I don't need to initiate any tactics - I just need to support the movements. It's a good one to learn the game as - even in the company of players who know it back to front - you, by following their lead, can actually be useful. No matter how raw you are elsewhere, no-one ever objects to being healed.
Second one is the key one and it's more subtle than it appears. Abstractly, everyone likes to win. If you ask - say - Alec or (especially) Jim whether they prefer being the one who gets bragging rights or not, the choice is clear. But they're not playing the medic. I play the Medic as I prefer to win than to have fun. Or rather it's impossible for me to have personal fun when my team is losing due to a problem I've noted. An impassable turret bottleneck? Troops when they've penetrated 2Forts are heavily wounded whenever they reach the basement? Medium-far from our spawn point in a war of attrition? That'll be a medic we'll be needing, and we'll continue losing until someone becomes one.
When I realise this, it annoys me so much I can't even faintly enjoy myself until someone makes the jump... and the easiest way to relieve that symptom is to swallow the pill myself.
In other words, the Medic is all about diagnosing the problem and prescribing the solution. Namely you and a magical healing squirty thing.
(And, yeah, the Engineer is similar, but we'll get to him eventu...)
For God's sake. I'm busy here. BUSY.
That's what you'll have to get used to as a medic. Constant demands for attention from team-mates, whether they're injured or not. The Z, X and C keys are bound to the sub-menus of communication... but simply press E and you call for the man with the sticking plasters. A conscientious medic spends a lot of their time spinning around, trying to see if the shout is someone they've missed or someone suggesting you team up for an attack or just pressing it as a joke. Note I said "missed". To play a Medic well, you need a constant awareness of everyone's health in the area - as well as a knowledge when your healing isn't going to be enough to save someone (A constant beam on someone has a tendency to make people over-confident. It'll let you shrug off many attacks... but any decent turret will take you to pieces). This means that medics do best - and are most essential - on the levels with bottlenecks where you can skip between your team-mates in a half-second.
Remember what I said earlier about the Medic not needing to initiate any tactics. Not completely true. They don't initiate offensives but - at least on open server play - it's not really tactics until a medic tags along. With a healing beam between the medic and his charge, it's a visual symbolism of a symbioticism - we're working together, we're a team. The Medic is the truest of the Support classes, the most hands-on in that way. You're engaged in the combat in a way which the Engineer often really isn't, as he goes quietly about his master plans in the middle of a warzone. You're wearing those big gloves for a reason, because you're often elbow-deep in gore.
The medics' rewards are mainly mental. This reward is mixed with a mass of aggravation.
Basically, playing a Medic rapidly turns me (and probably you) into a curmudgeonly surgeon who'll make House look like Bertie Wooster. Because, you get to see people fuck up, just to annoy you. For example, you loyally follow someone into a base, only for them to get themselves killed in a really stupid way to leave you stuck behind enemy lines, with your occasionally-effective-but-mostly-not syringe-gun and wasted minutes of your lives. And obviously, the Spies. As Snipers are to the Heavies, Spies are to the Medic. While Alec suggested Shoot The Medic to be an effective tactic against a paired-up heavy-and-medic, in practice, Medics partnered to Heavies will be safely behind the corner the big guy's marching around. By the time our Germanic-chap follows them around, everyone will almost certainly be dead. Unfortunately, as a medic, you're looking in the direction of the person you're healing... or you're not healing. You may as well have WE WELCOME ALL SPIES, PLEASE MAKE YOUR KNIFE COMFORTABLE written on the back of your head.
But that's not worst.
The charge function isn't the biggest part of the Medic's utility, but it's certainly the sexiest bit. After healing enough (just concentrating the beam on someone who's on full health will work too, but slowly), you fill up your charge bar. When you click your alt-fire you, and whoever you're currently healing, will become invulnerable. It's the stalemate breaker, allowing you to form the core of an assault to clear out those pesky turrets or defenders. It's also a lot of work to get, in terms of time. If you die, you lose everything you've saved. At least a minute of charging is required before the bar signals it's ready to fire gloriously. You choose the recipient of your boon, set the beam and click the alt-fire... and you're a pair, pushing back the line. Or you're turning the attack back in a duo of destruction. Or you stand, slack-jawed, as they continue the paranoid cautious standard TF2 play instead of leaping in like a true hero, or - as has happened at least a couple of times - they crouch in the corner for the entire length of your hard-earned invulnerability, doing sod shitting fuck all.
The Medic isn't really about altruism. It's about trust. In that you, to be most devastating, have to put your trust in another player. No other role requires such an enormous investment of something you've worked towards for minutes into someone else. It gives the pair of you the chance to be great. It also gives the chance for them to carry on, unworried, while you eat your monitor in frustration.
Coming, for God's sake, I'm coming. If I didn't have to keep you all alive to win, I'd kill the bloody lot of you.