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 can 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 can get easily blown out of proportion and become big, hurtful lies. Passive-aggressiveness is likely a problem between you, especially on your partner's side; they might feel the urge to purposely mislead you when they're feeling angry or hurt, or even to leave out information just to watch you struggle without it.
What a terrible urge that can be! They certainly never wanted to treat you this way -- and probably never dreamed they could -- but something in the way you two interact leads to it naturally, unless you're both vigilant. Falseness in the opposite direction likely occurs as well -- you might fake concern for your partner's problems, because you just don't get where they're coming from, and faking it might be the best you can do. This sort of dishonesty comes from a good place, but it could be just as hurtful as flat out deceit. Instead of having blowouts over this, it's better to work hard 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.