Read this great lecture: "Scar Tissues Make Relationships Wear Out", by a CS Stanford professor John Ousterhout.

A few takeaways:

• there isn't a single spectacular thing that wrecks a relationship, it's the accumulation of little things that builds up: scar tissue
• if you want relationships to last, keep the scar from building up through communication and compromise
• be willing to listen to understand the other person's view
• if there's an issue, resolve it where there is zero lingering animosity
• Some people are too nice, and that wrecks relationships because they're willing to give in, avoid arguments, and not compromise, which is a bad idea.
• "Ask yourself, "Are you really, completely, 100% over this? You're giving in? No animosity? You're not secretly hoping that maybe they'll do something for you in return or a little behavior change here or there?"
• The flipside is bad too, if you're outarguing the person, you're nuking the relationship