Trust

Trust is an interesting thing. What constitutes a breach of trust, especially in the context of a couple that share a space?

It seems obvious that a lie is a breach of trust. Lies are bad, right? Any small child will tell you so. While glaring untruths are fairly cut-and-dry, what of a lie by omission? Still a breach of trust, or just annoying and inadvisable? In what level of detail are we expected to inform others of our movements, friends and activities in order to avoid the dreaded omission?
It probably depends on the context, but I think if any information that one would reasonably expect is omitted and that omission goes on to alter the decision-making process, it constitutes a breach of trust.

Relationships are a contract. Sounds detached but there you have it. I'll provide you with companionship and financial security for as long as we mutually benefit. Mutual benefit can be defined in practical terms; money, or getting booty, convenience - it depends on the relationship. However, many romantic relationships in particular, but friendships too, can be and are defined also in emotional terms. Being "in love" is often a term of serious romantic relationships. That is to say, if one or both parties feels that they are no longer in love with their partner, they are entitled to end the relationship. Other common terms of romantic relationships include fidelity - if it is found that one partner is not monogamous when the understanding was that they ought to be, is that enough of a breach of contract to end the relationship? If the understanding was that sexual partners outside the relationship are permissable given all parties are aware of the situation and it is found that one person is having a sexual relationship without informing their partner, is that enough?

How about privacy? It's something we often don't articulate, but probably should in the interests of clear boundaries. Privacy seems easier to manage in separate houses - I won't rummage through your shit and you'll give me the same consideration. But it's much the same in a shared space. The main difference in a shared space is the likelihood of accidentally coming across something in the course of a normal day or while searching for something else. Letters, CDs, phones and laptops are all things that can lead to questions about the appropriate way forward.
Here's my rule:
If it's not addressed to you, then put it away.
Is that so hard? If you're looking on your partners email account for something with their permission, do it and log off. Don't read anything else. And for God's sake, should you find a very personal letter addressed to your better half and it is immediately obvious that it was never meant for you and that you have no business whatsoever reading it, then DON'T read it, give it to her and if you're dying to know ASK. Respect is something earned and I guarantee you'll get points for doing the big-kid thing and asking permission rather than coming off as a snoop.

Trust is, to me, like respect. Everybody starts at a base level and works their way up, some faster than others. It takes a long time to build, and only one stupid act to tear it down.

I understand the temptation to snoop. Anything from simple curiosity to suspicions of cheating can lead someone to take a peek at something they know they shouldn't. I know there have been times that I've been left with someone's phone or Facebook had been left logged in, and there is a definite temptation to have a look around. It might be just curiosity but even so I find it important to respect the privacy of others as they should respect mine. I'm not hiding anything, I'll answer any questions put to me, but I'll answer on my terms, not through having an uninvited spectator sitting in on private conversations.

This is why it angered me when, on briefly explaining the situation to my housemate, she answered that I shouldn't be upset at someone reading something they were well aware they shouldn't be because she does it to her boyfriend all the time and he's weird for not doing it to her. Why does this anger me?
Because while I'm not knocking what goes in her relationship, that shit does NOT fly with me. Last I checked, my relationship is based on mutual respect, and (to me) respecting personal boundaries goes with that territory. I do not believe I am alone in this.

So the way forward? Be clear with your expectations and expect them to be met as you strive to meet those expectations in yourself. Mistakes will always be made, but trust and respect are too precious to squander.