In the Zhuangzi, Confucius asks why, in his years of trouble, his friends have drifted away and his disciples have scattered. He is told that those joined by profit abandon each other in hardship, while those joined by nature take each other in.
“The friendship of a gentleman is plain as water. The friendship of a petty man is sweet as sweet wine. The gentleman is plain and so stays close. The petty man is sweet and so breaks off.” (君子之交淡若水，小人之交甘若醴)
“Those who come together without a reason,” the passage says, “part without a reason.”