Selecting the wrong mate!
Most relationships come about by happenstance or impulsive decisions. Very few people do any sigificant introspective thinking when it comes finding love and romance.
What ends up happening is they try and fit square pegs into round holes or change water into wine after they have become emotionally invested in someone who was not right for them to begin with!
You have to know who you are, what you want, and what you desire in a life mate. Once you have that down you can seek out another person who wants the same things you do and agrees with you as to how to acheive those things. (Both people have to want the same things in the same way). If one person always wants to go right and the other wants to go left then you're not going to see "forever after" together.