Whether it is in marriage or in a business collaboration, two people will find it very difficult to work out things unless they see things similarly at least on the core issues of goals, strategies and management. That is why the Bible tells us to be careful when it comes to serious relationships (2Co.6:14). "What partnership have righteousness and lawlessness, or what fellowship has light with darkness?" asks God. Let's take a look at these issues that can be critical in a relationship.
All of us have inherited a sinful nature from Adam and Eve when we were born into this world (Ps.51:5). This gave all of us an inborn inclination towards sin. We notice that as soon as babies grow up they start fighting, telling lies, acting selfishly, etc., even if their parents are godly! Later, people become self-seeking, pleasure-seeking, power-seeking, etc. Adam and Eve were thrown out of the Garden of Eden as soon as they sinned and lost their innocence, and the holy God could no longer have fellowship with them. He cannot have fellowship with us too, as long as we are in that natural condition.
But God then made it possible for us to be born again, receiving into our life His Spirit who gives us new desires and a new direction (Ez.36:26). Now we no longer like sin, and we want to be godly. From then onwards we seek to give up our old sinful ways and learn to live the way God wants us to (Ga.2:20).
What will happen if we get into a relationship (marriage, business, etc.) with someone who is not born again? He or she is still in the old sinful nature and cannot think in any other way than in their self-seeking ways. Ethics of society and the laws of the country may keep their excursions bound to some limits, but in their core, they are not seeking to be godly.
If we don't take this factor into considerations, we will be asking for trouble, which may surface only as time goes by. If it is a marriage, it can become truly miserable, and it may even end up in divorce which is one of the most hurting experiences we can ask for ourselves or the children.
It is in God's love for us and wisdom that He tries to warn us so that we won't get hurt later. But it is a sad thing that many get fooled by looking only at someone's external behaviour and not realising what is lacking in their core. When people 'fall in love' sometimes they say, "This person is actually better that even some of the Christians I know!" Ok, as Christians we struggle with remnants of our old nature and we overcome only to more or less degrees. Some of our failures may be even obvious. But the Holy Spirit is still working in us and we aren't going to remain the same for long! But in the case of unbelievers, they may be able to put on an impressive show just to win someone's heart, but in the long run their core cannot but show itself outwardly too. By then things can be too late!
God's commandments are for our protection and to lead us to the best in every way.