Friends and family
A massive part of your life is made up of your friends and family and, like it or not, the same goes for her. So, not getting along with the other people who share your partner's life can push any relationship to its breaking point. It's impossible to love everyone she knows, but maintaining "friendly" relationships with the closest people in her life (like her sister and best friend) is important in keeping your own relationship afloat. The long-running disagreement over politics with her father that has turned serious, the fact that you and her best friend can't be in the same room... at some point, she'll have to choose, and there's no guarantee she'll side with you.
Often, being part of a couple can put you in a "bubble" that closes you off from the rest of the world, giving you an us-against-them mentality. However, when the real world comes crashing into your little bubble, things can go really wrong. Issues like money problems, children from previous relationships, and issues carried over from past relationships, are possible relationship killers that might place strain on you. Even little domestic issues (one person refusing to clean or cook) can chip away at your coupledom.
A couple is, by definition, made up of two people. Throw a third person into the mix and things will start to go wrong. If someone else if trying to get with you or your other half, distrust and suspicion can breed quite easily and can be a serious relationship killer. The effect of this is increased if the "other person" is a lingering ex. If the issue should arise, deal with it as quickly as possible. If you're the one with the admirer, simply tell her you're not interested; being friendly with her could give her hope and permission to stick around longer and toy with your current relationship.
We already know you can't like everyone all the time. Well, sometimes this counts for your partner too. Everything about her might be great, but if she has a few annoying habits, be careful. Stupid and trivial as these annoying habits might be -- like talking in her sleep, leaving the cap off the toothpaste and wearing too much makeup -- they can be relationship killers and wreck a couple. Over time, these habits become magnified until you're angry with her more often than not, and you've talked yourself out of a relationship.
High comfort levels
After being together for a while, couples tend to grow comfortable with each other -- way too comfortable. They become more like friends than lovers as intimacy becomes way too familiar and routine, and they start to let things slide. Sex falls off the agenda and, before they know it, they've become the person stopping the other one from dating someone else.
Over time, if a couple isn't careful, they might begin to drift apart. This could be due to a lack of communication as other things, such as careers, become more important, or it could be a symptom of a relationship that just isn't working. If both people are chasing separate goals, such as one wanting to put down roots where they are and the other going after a promotion which could involve relocating, clearly their future is in jeopardy. When this situation is left unchecked, someone is eventually going to have to choose between their ambitions and their significant other.
People who live in the past find it impossible to move forward. If one of you insists on comparing your new relationship to previous ones, the future doesn't look good. While learning from past mistakes is a good thing, treating them as a blueprint for every other relationship is not. What you have now is different to anything you've had before, so let it grow free from past screw-ups. Likewise, dragging the past of this relationship into the present is also damaging. So what if one of you messed up in the first few weeks? There's no point mentioning it in every single argument. If a couple decides to move on from a problem, they move on and focus on the future; the past should always be left in the past.
Moving too fast
Healthy relationships progress naturally. While not always at the same speed, they tend to move on as both people become more and more comfortable with each other. It's an unspoken, instinctive thing. However, some people seem to lack this instinct and rush to make a blossoming relationship into something it's not ready to be. No one likes to be rushed. A couple that sees one partner constantly pushing the other to commit before they're ready is a couple that won't last long. Especially dangerous is the M-bomb. Once one person is chasing a wedding the other isn't ready for, an otherwise successful relationship is usually over.
Maintaining the right balance between dependence and independence is tricky. Too much of one and people feel smothered. If a couple gets to the point where they have nothing in their life apart from each other, they need to back away a little. Otherwise, one partner will feel the need for space and resent the other for taking their freedom away. Conversely, if too much independence is asserted, the other person starts to feel lonely. If one partner has an all-absorbing job or interest with nothing but the minimum of time for their partner, things are equally bad. While space and time apart from each other is needed, too much is as bad as smothering -- both end with someone looking elsewhere.
Cheating is the ultimate relationship killer, and one that the majority of couples won't survive. The initial betrayal of finding out that a partner has cheated is often enough to leave a couple stone-cold dead while the bed sheets are still warm.