I can think of at least 3 things.
The first is a misconception that all started with a silly line from a movie-
"Love means never having to say your sorry".
That is really not true. If you love someone, you will need to say I am sorry a bazillion times in your relationship because you are going to do things that are hurtful.
The second thing is pride.
It is hard for us to admit when we are wrong. Somehow we cling to our pride like a badge of honor. My mother always told me it takes two to make a fight. If you are one of the two, swallow your pride and admit your part in the wrong (sincerely).
The third comes from being stubborn or arrogant.
One of the worst things I hear people say is, " By god that is just how I am and I aint gonna change. If they don't like me the way I am they can just..." Well, you get the idea.
We NEED to change. We start out life as a baby and we change and grow all the way into adulthood and then one day decide we are done. Not true!! It is as an adult that we can really learn to grow-- and grow well.
Being stubborn and not being sorry because that is just how you are?? That may be how you are- but that really isn't such a great way to be.
Swallow your pride, soften your stubborn heart and quit taking advice from movies (Hollywood is the last place to seek relationship advice).
Just say I am sorry.