clarke literally fell from the sky and fought a war on a radioactive planet that wiped out civilizations. lexa has been trained to be a warrior since she was 2 and trying to maintain a coalition she created that could break at any moment.
like the chances of both of them surviving or even existing at the same time and falling in love with each other was too small, almost improbable, but they did. they fell in love while the world fell apart.
their love is so pure and they respect and understand each other on such a deep level it just makes it more tragic that they live in a world where they have to make really hard choices.