You have to force yourselves to be completely honest and clear with each other; otherwise, the lines of communication become entirely too muddied. With other people, you could afford to fib every now and then -- for example, to avoid hurting their feelings when your opinions are just too different -- but between the two of you, these small, innocent fibs are blown out of proportion and could become big, hurtful lies. Passive-aggressiveness could become a problem between you, especially on your side; you might feel the urge to purposely mislead your lover when you're feeling angry or hurt, or even to leave out information just to watch them struggle without it.
What a terrible urge that is! You don't enter the relationship wanting to treat your lover like this, but something in the way you two interact leads to it naturally unless you're both vigilant. Falseness in the opposite direction could occur as well -- your lover might fake concern for your problems, because they just don't get where you're coming from, and faking it might be the best they can do. This sort of dishonesty comes from a good place, but it could be just as hurtful as the more intentional and negative kind. Instead of having blowouts over this, it's better to try from the start never to mislead each other or cover anything up. If you don't understand where your lover's coming from, just say so, and go on from there.