Memento mori: remember you must die.

His father had told him, before he was old enough to understand what it meant.
Sherlock always thought it sounded lovely. Latin was like that.
The way the sounds flowed over one another.
It was like music.

When he was old enough to find out what it meant, it didn't lose any of the beauty or appeal.
He often wondered why Latin was a dead language. It was so pretty.
So he kept the phrase close to his heart.
Lived by it.

Sherlock forgot that life was about balance, about light and dark, life and death.
He was shocked to find the other half of his motto.
It sounded just as pretty.
He loved it.

Memento vivere: remember to live.

John loved it.
He'd first read it in university.
Latin was all over medical textbooks, and he'd become intrigued.
Other Latin phrases appealed to him, and this one was no exception. Perfect for the doctor.

He was shot.
It reminded him why he lived by it. (Then made him forget.)
When he came home, he couldn't listen to it. Couldn't remember why he had.
How was he supposed to remember to live when death was always waiting in the wings?

Life wasn't alone.
Death always travelled close behind.
When John met Sherlock, they both found the perfect balance of the two.
Sherlock reminded him that they'd have to die, and John reminded him not to forget to live first.