Expectations
Have you seen those videos of conjoined twins?
Even if you don't know the term I'm sure you've seen them...their bodies are inseparable at the hip of hand or back.
Sometimes, they show them happy on the screen but it's a difficult journey to be on. Whatever one does affects the other.
Same is with relationships.
Unless you can agree with your partner on making major decisions, it can be a difficult journey.
One of my friend had his to-be wife quit her PhD in order to move to a new city where he could restart his career. This decision rewarded him beautifully...financially and otherwise but on the personal front, directly or indirectly, that ended up being the reason of a of fights amongst them and ultimately divorce.
This might seem a bit far fetched but little things add up to big ones.
We often assume how unsettling it can be to take big steps.