lie and cheat and hide things from eachother.. :)
making assumptions and accusations without FACTS
Pretty good question. Here's the view from the "rearview mirror"; for men.. understand the she will have needs that do not come instinctively to you. Remember to pay attention, and give her whet she needs, always, but not necessarily everything that she Wants. For girls; Do NOT make the common mistake of viewing your boyfriend as a "diamond in the rough" and think that your love will change him. Stupidest mistake that young women make. Let him be who he is, or he will be miserable, and you will not hold him long. Celebrate who he is, not who you'd like him to be.
Not being genuine and open and honest, from the beginning.
They do not know their personality types nor those of the person they fall for.
Not realising the needs of the partner...
By starting to analyze what their partner says and does instead of just naturally coexisting with one another and doing things as they do them.