I know that I tend to avoid touchy-feely interpersonal topics, but here I go anyway....
To my eye, there seem to be three stages for any couple:
Stage 1: Hanging Out (AKA dating)
This stage is often full of drama, but the only people who really care and matter are the two people who are hanging out. Part of the point of this stage is ascertaining whether this couple should move on to the next stage, but that answer is often knowable early on, so most of the point is just about hanging out. When it doesn't work, you can just say "I break with thee, I break with thee, I break with thee" and throw dog poop on the other's shoes. Easy peasy.
Stage 2: Shacking Up (AKA getting married or living in sin)
At this stage, you find out if you are able to wake up next to the person every day. Can you figure out who does laundry and dishes? What food do you like? What shows do you like? Do your sleep schedules work together? At this stage, the stakes are higher than in stage 1: If it doesn't work out, then there is more emotional trauma and one or both of you needs to find a new place to live. Still, even if lawyers have to get involved, a complete breakup can be achieved in just a few weeks or months.
Stage 3: Being a Team (AKA Raising Kids)
This is where things get serious. Raising kids casts a bright, cruel light on your differences. If you and your partner don't share basic values, then raising kids together is going to be a challenge. I know couples who were great at stage 1 and stage 2, but then didn't do well at this stage. It's not that they were perfect couples that fell apart (no couple is perfect), but raising children has a way of discovering and amplifying problems. Opposites can make stages 1 and 2 really fun, but at stage 3 having unified values and big-picture strategy is key. Kids are common ground that cannot be divided.
I am amazingly lucky. I happen to be part of a tremendous team. By this I mean that I am mostly holding a clipboard while the coach and franchise player (Mrs. Kid fills both roles) do all the thinking and the real work. But seriously, we are in sync on just about every major decision (and even most of the minor ones). We have the same goals and fears for our kids and that makes everything else fall into place pretty well.
I have great sympathy for my friends who have found that their teams don't work so well. I tend to not blame one person or the other: It generally takes two to make it work or to get into a situation that does not work. Still, I feel for everyone involved.
[update: I realize that there is a fourth stage: denouement (AKA empty nest/retirement), but have no experience with this final stage, so I'll post an update in another couple decades]