Simply because there is no perfect relationship. No matter how much you see yourself in your partner, the truth of the matter is that you are two different persons, with different ways of thinking, perception, and opinion on things. How do you think relationships last? It is because they are able to overcome all these differences and choose to be together despite having different ways of thinking, perception, and opinion on things.
However, there may be times when the fighting would be too frequent, and I think that's the time you need to reflect if you two are best suited together. I won't advise you to stay in a relationship if it's so toxic and is draining the life out of you. So if the number of days that hurt are far greater than the number of days that don't, then I think it's time to leave.