Because you like Dana Scully in The X-Files
A few millennia in the mortal realm have caused DEATH to be quite enamored of life. See them in Discworld.
Angel is fighting as best as he can, but the disease might be too powerful for even his strong, positive energy. See them in Rent.
Fighting for a future without fear? Yeah, Chris Redfield believes it is worth it. See them in Resident Evil Series.
Clarice Starling is patiently working with one serial killer to stop another. See them in The Silence of the Lambs.
Although Morrigan knows every kind of magic there is, friendship is an untested area for her. See them in Dragon Age Series.
It's hard enough fighting a war. But it's worse for Commander Shepard, knowing no matter how hard you try, you can't save them all. See them in Mass Effect.
When a half-breed breaks the rules, John Constantine deports them straight back to hell. He's trying to buy his way into heaven. See them in Constantine.
Blade has all the cool vampire powers with none of the weaknesses. Definitely not a guy to pick a fight with. See them in Blade.
Despite the war-torn world around him, Mal Reynolds still holds onto the shred of hope that life can be more than just living another day. See them in Firefly.
Shaun is used to being lazy. And now, you're asking him to fight for his life? See them in Shaun of the Dead.
If anyone can stay optimistic and friendly under harsh alien rule, it's Alyx Vance. See them in Half-Life 2.