Proverbs 27: 17 "As iron sharpens iron, so people can improve each other."
"Each other" literally means "companion or friend; neighbor or associate". God uses every relationship to improve us and to accomplish His will in us. People come into our lives for eternal reasons. Relationships are mirrors. In them, we see reflections of ourselves.
Relationships expose the weaknesses we try so hard to bury and teach us important lessons like forgiveness, patience, kindness, goodness and self-control. If we need to learn patience, He brings someone that is irritating. If we need to learn to love, He will bring the unlovable. God takes those people we call "difficult" and uses them as pruning tools, tools of molding and refining. If we fail to learn from one, He will send another. Even if we do learn from one, He will send another to teach us a different lesson.