Nothing gold can stay

There's a poem by Robert Frost which ends with the line "nothing gold can stay."
Basically, all good things come to an end.

This echoes a question I've had for a couple weeks now:
Why do the best characters have to die?
I've heard it multiple times that a character was "too good to live."

Okay, getting into the realm of spoilers, so beware.

I'd never even questioned it until I was talking to my friend about Tadashi from Big Hero 6.


He's an awesome character. Really awesome. Probably the kind of guy most people would like to know.
And his end is freaking tragic.
(As a predictable aside, my favorite character died very young as well, though not as young as Tadashi)

I've killed the "best characters" too. In Evva's story, my favorite character dies. He's not a human, so I felt like I could get away without giving him a lot of human faults.
And I knew from the start that I would have to kill him off because he was too good.

But why is that? Why is it a given that the best characters don't make it to the end of the story?
What are we trying to tell ourselves by killing them off? Do we think that no one that good could survive in a messed up world? Do we think that no one that good could even exist? Are we telling ourselves that we don't deserve to know (or even be) people that are just great and awesome?



I mean sure, we can't expect everything to last for ever, but that doesn't mean that everything good in the world will vanish before we do.
Maybe some things gold can stay.