Why does it happen? Because there's nothing anywhere that states that life is fair.
I'm truly sorry for your loss, but one of the things I've always hated about watching those I love suffer extended deaths is that you can't actually mourn the person and move on until after you die. So, it wears you down a lot in the process.
You might also check out the book: [URL="http://www.amazon.com/Survive-Loss-Love-Peter-McWilliams/dp/0931580439/ref=sr_1_1?ie=UTF8&s=books&qid=1265945155&sr=8-1"]How to Survive the Loss of a Love[/URL]
But, grieving, time, reflection, and generally reminding yourself that life doesn't stop for you when someone you love dies. You still need to go through the motions of caring for yourself.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."