Couples who live together before marriage are more likely to get divorced than those who don’t. There must be some powerful reason for that, since every “objective” or “logical” evaluation would seem to say that the results would be reversed - that two people should live together and “test it out” to see if they really are compatible - that those who do should have a much lower divorce rate. It’s fascinating that it doesn’t work out that way.
I see a great "deception" in living together, since really all it does is lengthen the “courtship” - the time when you are putting on your best face to try to impress the other person. You get married thinking you really know the person, but all you know is the person you simply have been dating more intimately. When you get married, your guard gets let down and you realize the person you lived with isn’t the person to whom you now are married - and it can be crushing. You feel betrayed and deceived - whereas those who have not lived together are much more understanding of the change. Often, those who don’t live together are even told something like, “Just wait until you see him/her after you get married . . .”
There also is the aspect of trust and commitment: one couple is firmly committed and focused on the positive, while the other is almost looking for reasons not to get married. Even if marriage occurs, it’s hard to change that basic mindset and perspective. It’s hard to look for faults and reasons to avoid marriage for an extended period of time, then stop doing so once the wedding occurs.
Most people don't stop and think about that - the habituation of skepticism and critical evaluation (which is a topic for discussion with many more issues than just cohabitation and marriage).