Cheating is definitely one quick, big way to annihilate a relationship, but there are plenty of other things that are much more potent when it comes to ruining even the strongest of bonds. Some of the worst things that lead to a break-up (that aren't cheating) start out small and indiscernible, and that's what makes them so dangerous.
1. Lying to and hiding things from your partner. Even if you're doing it out of love to protect their feelings, keeping little things from someone you're dating can grow into a big problem and cause trust issues that wreck a relationship.
2. Withholding any kind of affection. Whether affection means being touchy-feely, asking each other intense questions about meaningful things, or helping each other through obstacles, it's a crucial part of a relationship. Being absent and unaffectionate can cause the kind of doubts in a relationship that end up being irreparable.
3. Harbouring quiet resentment. It starts out quietly but becomes something big over time, which is what makes it so damaging.
4. Lack of communication. This doesn't mean not texting all day, every day, but failing to talk to each other about things that bother you when they come up, or conveniently leaving out details that you think might cause problems. It all just gets bottled up or revealed eventually, and by the time that happens, it's usually unmanageable.
5. Being stubborn about things or getting entrenched in certain positions. This is just another way of saying 'refusing to compromise'. If you like somebody, you should be willing to make allowances. Partners who can only handle things if they go a certain way (their way) are basically just in relationships with themselves.
6. Bickering about mundane daily issues and chores. So the argument you always have about whose turn it is to take out the bins seems like no huge deal now, but that's the sort of thing that just becomes a bigger issue over time. And even worse, it becomes ammunition for bigger, more serious arguments down the road.
7. Condescension. Talking down to a partner is just another way of making yourself bigger or more powerful than them, and a power imbalance is the last thing you want in an enduring, healthy relationship.
8. Staying in a relationship out of convenience. Whether it's because you don't want to disappoint your families with a break-up or because you have a big trip planned in six months, staying together just because you feel you should only leads to bitter resentment and an inordinate amount of fighting and heartbreak, when there was a chance you might have been able to stay friends.
9. Manipulation. This is tricky because it's often so subtle, and you don't realise you're being manipulated by your partner (or doing the manipulating) until it's too late.
10. Jealousy. Even without actual cheating, just the suspicion that it's always happening can be much, much worse.
11. Presenting a false version of yourself at the beginning. This can be as simple and small as pretending to like horror movies when you actually hate them, or as big as saying you're not looking for anything serious when the only thing you want is a long-term relationship. It's best to be up-front from the start because those little things can become huge reasons to break up over time.
12. Staying together because you've become co-dependent. Neither of you wants to be single. While you don't have chemistry any more, the only reason you're together is so you don't have to be alone.