Richard, I think you really nailed it. At the end of the day it comes down to being a power thing - manipulation and trade-offs.
I see a lot of comment about how you should weigh up the other person to see if they are that perfect match, but surely the reality is that all relationships are about compromise, about growing together over time and tolerating differences. The time a relationship goes wrong is when one or both are no longer prepared to compromise and tolerate, leading to a growing apart and eventual polarisation. In other words, one or both have given up on each other. That may happen sooner rather than later, but surely to put tight rules on a relationship from outset is a good way of ensuring it fails sooner.