I remember back to my less-than-glory-days of being single. A little over 30 years ago, I was in college, and one Sunday evening I was at the home of Pastor Wayne of the church I was attending in Lynchburg, VA. Not only was I single, I had somewhat recently gotten serious about learning theology, but of the “super-spiritual” kind; I hadn’t yet wrapped my mind around the in-real-life kind.
Wayne asked me about my relationship status, and I acknowledged that I was not married and that I was thinking maybe I wouldn’t ever get married. This answer is a typical and immature effort to present oneself as being content so that God will see your contentment and bring the spouse you’re not-so-secretly longing for; it also wouldn’t hurt if the right “someone” happened to overhear how content you are and realize you Made For Each Other. That perspective has its own problems. But Wayne smirked and said, “Well, you need to get married so that you’ll realize how much of a sinner you are.”
This isn’t because your spouse takes over for the Holy Spirit and follows you around to tell you how wrong you are all the time. This is because sin is always in relationship, first to God, then to others. Family relationships, which God invented and arranged to start with spouses, provide daily heart tests. The Scriptures are a mirror (James 1:22-23), and your spouse (or your siblings) will be sort of like lights around the mirror that reveal where you need to wash your face, so to speak.
exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin. (Hebrews 3:13 ESV)
Your spouse is a daily means of God’s grace in your sanctification, a constant reminder—one way or another—to keep your heart soft and your loves in order.