Death
When Diana of Themyscira came to Man's World, she had many things to learn. She started from the basics then, as if she's just been born.
First of all, she learned about men, and that they weren't as evil and cruel as she'd been told on her island. In fact, there were many honourable and charming males and she admitted that they intrigued her. Especially the one, who always hid in the darkness.
With time she learned how to live between people and how to communicate with them, or at least try to do so. She was studying people's habits and traditions. She looked at the humanity with curious eyes.
She learned that Earth needed its heroes, needed her. She realised why it did and as time went by she got attached to Man's World.
She was learning fast, using that magnificent mind of hers. She wanted to understand everything she was seeing and discovering and it came to her easily. Until one point.
Death.
As the Princess of the Amazons, Diana was blessed by her gods. She had Amazonian pride and strength. She'd gotten wisdom of Athena, beauty of Aphrodite and spirit of Artemis. However the thing that really distinguished her from the others in this world, the thing that didn't let her understand death, was her immortality.
She knew, of course, what death was. She knew it took many shapes and forms. She was aware that it was all around, while she continued living in Man's World. She knew she didn't bring death to anyone and that she fought with people, who did.
But still, she didn't understand death itself. She didn't understand how it felt, how it made people feel. She didn't understand how it made him feel.
He wasn't like the others. Not that the others would ever cross the line, which separated them from killing. None of them would ever do this. They were heroes, the honourable ones, killing disgusted them. But when it came to him, she knew, it was deeper. He didn't kill, but not only because it was against the law, against the morality. He didn't kill, but not only because he was a good man. He didn't kill, because he knew how death felt.
It was painful knowledge, and maybe he wished he didn't have it; nevertheless he understood.
She did not.
She tried to ask him, tried to learn what death meant to him, but soon realised that he couldn't just tell her. No one could.
"I hope you'll never know how it feels, Princess," he told her once and Diana didn't ask anymore.
She continued to live her immortal life. She fought with causes of death and saved people from it, but she still didn't understand. For her, life was going to last for many centuries or millennia, maybe even to the end of the world, and it wasn't her fault.
It happened many years later, when she finally comprehended.
The worst battle they had ever had. Many injured people. Many badly injured people. And he was one of them. Diana never in her entire existence felt such fear and helplessness.
And it was when she realised she was learning.
When it was his life, thrown at the edge of death. When it was the man, who'd become the part of her soul, fighting for surviving. She'd never thought that learning will be that simple. She'd never thought that there will be only two words in her lesson.
But there were.
Two words, spoken by Clark after he walked out of his room in the hospital; the expression of pain and sorrow on his face.
Two words , that made her understand death.
Two words, that made her know how it felt.
Just two words.
"He's gone."
It was when she understood.