It's very easy, IMO, for those who have it to say that everyone can. However, there is simply too much involved in creating a successful relationship between two people, and some of those things are very minor things. Has there ever been a person that simply bugged you? I mean, have you ever known someone that annoyed you just to be near them, and you didn't really know what was at the base of it? All too often, it's little stuff. It could be that they remind you of your father, 4th grade teacher, priest or whoever, that you'd had some really bad experiences with before, and so you are rejecting that person because of the shape of their nose, or the way that they push their glasses back up, or perhaps it's some other inconsequential thing they do that in and of itself is nothing, but because it reminds you of the one you had that bad experience with, you will most likely never get to know that person, because of something that (really) is wrong with you (It's not that person's fault that they remind you of someone else!).
Those same sorts of things can start to show up in a relationship. We as people are actors. We tend to act one way with family, another way with friends, still another around those we are attracted to, and still another way with those that we are in love with but only have been for a short time, and still another with those we have been loving for years. And, when you consider that it's not just one person, but two (or more), in a relationship, that is a lot of acting and hiding the truth, and it's a lot of changing, which requires a lot of adjustment for both (or all three or four or...) people to make, and with any acting, there are people that are good at it and there are also not-so-good actors, which tends to bring forth distrust in the relationship ("What are you hiding from me??").
By the way, if any of you disagree with this whole acting idea, ask several of your friends and family if you act any differently around your family, your friends, those who aren't your friends, your co-workers, those of the same sex, those of the opposite sex, etc. You'll find out that you're as guilty of it as the next person...
"It is better to keep your mouth shut and to appear stupid than
to open it and remove all doubt."