One thing that was impressed upon my husband and I when we got married was the idea of selflessness. We were reminded to put each other first. Even for two like-minded Christians, one of the hardest parts of marriage is consistently putting the other first. I think it takes a marriage, with all of its vows and public commitment and promises made before God Himself, to truly nudge a person closer and closer to selflessness. It doesn't happen instantly or automatically, but I believe marriage is intended to cultivate selflessness over time.
Isn't this why Jesus compares the marriage relationship to that of the church and Himself?
22 Wives, submit to your own husbands, as to the Lord. 23 For the husband is the head of the wife even as Christ is the head of the church, his body, and is himself its Savior. 24 Now as the church submits to Christ, so also wives should submit in everything to their husbands.
25 Husbands, love your wives, as Christ loved the church and gave himself up for her, 26 that he might sanctify her, having cleansed her by the washing of water with the word, 27 so that he might present the church to himself in splendor, without spot or wrinkle or any such thing, that she might be holy and without blemish. 28 In the same way husbands should love their wives as their own bodies. He who loves his wife loves himself. 29 For no one ever hated his own flesh, but nourishes and cherishes it, just as Christ does the church, 30 because we are members of his body. 31 “Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.” 32 This mystery is profound, and I am saying that it refers to Christ and the church. 33 However, let each one of you love his wife as himself, and let the wife see that she respects her husband.
Our highest calling is service and submission to God, and the Bible tells us our entire lives are to be lived as a sacrifice.
Romans 12:1 I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship.
That doesn't sound like a magic trick that turns ordinary, selfish humans into reverent worshippers. It sounds more like God working on our hearts as we strive to live as He would have us to live. My point being, God wants us to see our lives as a living sacrifice long before marriage and children are involved. But I do think these things help teach us the concept of sacrifice like not many experiences can.
Certainly, if marriage has not yet caused you to take the focus off yourself, having a child will do it. Especially as a woman, pregnancy itself puts a tiny life inside of you, forcing you to consider even more carefully what you eat, drink, slather on your skin. Your body changes without your direction or input. Feet and elbows eventually find resting places between your ribs and hips. You may be tired or emotional or sick. And then this life that you have guarded is born and continues to be completely dependent upon you for every need. All of this is joyfully received by most mothers because babies are wonderful and we love them. Not much thought is given to what mothers are sacrificing to care for their babies (and toddlers and children, etc.) because what choice do they have? It is a sacrifice, but it is one that we are blessed by God to be able to make.
I could go on about all the hilarious ways that mothers are forced to realize that it is not about them (anymore) like the lack of uninterrupted showers, meals, trips to the bathroom... slightly embarrassing bodily functions, spit up on your favorite shirts, a restricted schedule... These are the things that well meaning friends and family members who do not have children tend to recoil at. And I always want to say "yeah, well once you have kids you'll realize it isn't about you anymore. And that will be ok, because you'll have this wonderful child to love."
What I really find myself wanting to tell them is that it already isn't about them. Maybe they wouldn't be so apprehensive about curtailing their weekend activities, and getting up a little earlier, not always getting to wear the latest style, not having the body they used to... if they knew that stuff already doesn't matter. Not that I think everyone needs to hurry up and have kids. But the transformation in my heart that has been so directly related to having children is so important to my spiritual well being. There is no worldly thing that I would take in exchange for the spiritual growth that I have experienced by having children. I know not every spiritual person gets married and has kids, and I'm sure God has other things in their lives that help transform their hearts in much the same way.
I am so thankful that God designed us so that we are always pointed back to Him. The monumental things in our lives turn us to Him. How much easier it is to be a living sacrifice when you know it isn't about you!