One can surmise from the text that George and Lennie, the two main characters in John Steinbeck's novella Of Mice and Men, have been friends since childhood and that they began traveling together since the death of Lennie's Aunt Clara.
There are a few passages which lead to this conclusion. In chapter one George makes reference to Aunt Clara after Lennie retrieves the dead mouse George has taken away from him. Lennie, because of his mental disability, can't remember who first gave him mice to pet. George says,
“Lady, huh? Don’t even remember who that lady was. That was your own Aunt Clara. An’ she stopped givin’ ‘em to ya. You always killed ‘em.”
In chapter three George gives the best explanation for the relationship between he and Lennie when he's talking to Slim in the bunkhouse. Slim says it's not often that he sees men traveling together. George explains,
“Him and me was both born in Auburn. I knowed his Aunt Clara. She took him when he was a baby and raised him up. When his Aunt Clara died, Lennie just come along with me out workin’. Got kinda used to each other after a little while.”
Later in that chapter George gives more evidence about how long the two have been friends while discussing Curley's wife and how she could get them into trouble:
“You remember Andy Cushman, Lennie? Went to grammar school?”
“The one that his old lady used to make hot cakes for the kids?” Lennie asked.
“Yeah. That’s the one. You can remember anything if there’s anything to eat in it.” George looked carefully at the solitaire hand. He put an ace up on his scoring rack and piled a two, three and four of diamonds on it. “Andy’s in San Quentin right now on account of a tart,” said George.
The fact that Lennie has been with George for so long makes the ending even more poignant. It would be hard to imagine killing so close a friend.