You might get along really well for a time, but then realize that you have less in common than you thought -- or that there's some sort of disconnect in your communication. There's a good chance that you both formed images of each other that aren't entirely accurate, and as a result, you don't know each other as well as you should or could as the relationship progresses. On your partner's side, their incorrect image of you is based on the fact that you aren't entirely straightforward with them about who you are and what you're thinking or feeling.
This could be as minor and intangible as simply holding a part of yourself back due to insecurity or past experiences, but the effect is the same -- they don't get a full picture of you, and they could be surprised when certain personality traits start to come out of the blue. And the fact that you got involved with each other before you really knew each other could mean that your expressions of affection are less than sincere, because you might not even be certain of your own heart! If you can both try to take things slowly and represent yourselves as honestly as you can -- even when you think the other person isn't going to like something you're about to say or reveal to them -- you stand a chance of moving past this difficult influence.