In 4E D&D, even though the opponent is adjacent to the defender and a number of his allies, that opponent will be punished if attacks someone else than the defender and, since 4E says that creatures are aware of any such situation, the opponent is consequently encouraged to attack the defender. The mechanic makes sense game-wise, I'm having trouble wrapping my head around the concept making sense for the individual living the defender's life.
You're a Fighter. Let's say Sword-and-Shield. You have trained in the Guardian School. You know you're tougher than your allies, and better defended, and you can't bear to imagine standing there, at the end of the fight, cowering behind your shield while they're dead.
But, as I say, you're trained in the Guardian School, Mamorujutsu, a martial art that uses a blade to defend your allies.
You want the enemy dead, or at least not fighting anymore. So, you go up to one of them, a sneaky looking bugger with a rapier, and you "mark" him: you watch him, paying attention to his every movement.
The moment he takes his eyes off you, BAM! you strike at his exposed flank.
He turns back to you, and you clash swords. He's learnt his lesson now, he never turns away from you.
You're not worried about your own safety, you'll be fine. You've got your armour, your sword, your shield. You can take a few hits.
But William the Wizard? He's a good guy, really, his spells really help in a fight, you love him to bits. Without him, you'd eventually be over-run by sheer numbers.
But he's a nerd, with a book, wearing robes. If this bugger got to him he'd only last a few seconds.
So you stand here, and you guard him while he prepares his spells, and blasts through the enemies.
But then, Robby the Rogue gets caught off guard by a massive brute of a man, slammed down to the ground, he's nearly down for good.
And the sneaky bugger you've been fighting looks at you, grins, and tries to dodge deftly past you. If he gets to Robby there won't be enough left of the poor boy for Clara to pick him back up. You act instantly, without any conscious thought, and your blade digs into the shoulder of the sneaky bugger.
He stops himself before plowing himself full into the blade, but only just. He's not going to get to Robby.
He turns, enraged, and strikes at you again. This time your shield is just out of place, and his blade hits flesh.
That's the cost of defending your friends.
But you're tough, you can take it.