Some have just flat been beat by life facing issues like financial pressures, loss, health problems and demanding work schedules. Since the Genesis account of Adam and Eve, I believe the evil one has had the guns of hell aimed at the intimate bond of marriage. He is the great confuser and the ultimate liar. He magnifies our weaknesses and fears and wedges them between us. And, he's out to take as big a bite as he can out of your marriage.
Unrealized expectations breed major disappointment.
If our expectations are unrealistic for our marriages, we're setting ourselves up for a fall. Marriage will complete me. Perhaps we grew up with parents who didn't care like they should, or with siblings who stole the limelight, or in some other painful environment. We may expect marriage to reverse all the negatives we're carrying in to it. My spouse won't hurt me.