I've heard that the person we marry (or commit to, in whatever way) is actually three people. This, I've found for the most part to be true. Three people? What do you mean? Yes, three people. The person you're in a relationship with is really: 1) the person you imagine them to be. 2) the person they really are. 3) the person they become as a result of being with/living with, you.
About person #1- In the beginning of a relationship, there are several things that each of you do that might annoy the other, but you find yourself thinking - oh, that's ok. You imagine they will change from doing that, or you will always feel as though- that's ok. (but they don't change and you won't always be ok with it). You hope and imagine the other person to be a little more perfect for you than they may actually be.
About person #2- At first, you're both on your best behavior. You each try to look your best, act your best. You're both at little more polished, a little more alert, aware, polite, considerate. Normally, you let your moods show, you might be more selfish, less polite and considerate. You each wear a label that may not necessarily represent what's inside. You may not be presenting yourself as you really are.
About person #3- As you realize you are part of a couple, you each change your habits, your priorities at least slightly. As a single, perhaps some routine was very important to you. Now the other person has pointed out that either they would rather be doing that routine not you or that there's no need to keep doing that or even that something else needs to be done instead. Perhaps squeezing the toothpaste tube was not something you normally did. Now your mate has asked that the toothpaste tube be squeezed. Perhaps making the bed each morning before leaving for work was important to you. Now your mate gets up later, the bed's not made, it doesn't matter any more. Perhaps you never cooked, now you do. People change, adjust, adapt to living with someone else.
So each of us brings three of us into a relationship.