The first thing everyone needs to do before merging their life with another is figure exactly who they are and want they want from a mate. If you don't at least have a mental list of the traits and values desire then it's easy to let "chemistry" or "circumstances" sway your decision making process.
Some folks have actually gotten married simply because "it was time" or they felt it was "the next step" after being in a relationship for a while. Others marry someone knowing this person is not their ideal mate but they believe he or she has "potential". When it does not pan out they become frustrated. I've only heard of one person who was able to "change water into wine".
If you go to the store to purchase an apple but buy an onion instead...Whose fault is it? The key is to make (your own shopping list) and then have the discipline to stick to it. Not knowing what you want up front can lead a person to settling without realizing it. One other point worth mentioning is there's a difference between "settling" and becoming "realistic" about one's expectations. No one is "perfect".
Having said that it also is possible to know what you want at one age or stage in your life but overtime your desires change. You have to express/communicate your wants and check in with your mate to see if they are open to moving into the same direction. Whether the answer is yes or no it's important to remember, "You are responsible for your own happiness". If you want something different you have to do something different.