I'm no expert, this is my opinion only.
I'd say the first one is a lack of communication. Followed by money (too much, not enough, ensuing rows about) and children.
It's easy to get caught up with life's little details/difficulties, and to avoid ironing out relationship glitches - that eventually grow and grow. Plus money can become an issue - one partner spends too much, doesn't earn enough, too much month at the end of the money and so on.
And if and when kids come along, tiredness kicks in, the children have to have their needs met, partners can lose their way with (or sight of) each other.
Relationships require a lot of input and I honestly believe talking is the best solution, finding the time to do so, being honest with each other.
Be good to see what others may think :)