It does appear that some people are a tad more suited for
marriage than others.
Some couples get hitched, and then they fall into their new roles
comfortably. Better still, they have the same goals of having kids,
starting their own families, and their value systems tend to be
similar so other than a few hiccups along the way, which is
inevitable in any relationship, they seem to thrive well as a
couple.
Others get hitched without really understanding the full
implications of it, and are thus not really prepared for the
ensuing responsibilities, roles and expectations. They start to
feel stifled / get scared / feel inadequate or overwhelmed, and
then they start acting like a frightened, cornered and captured
animal! 
Also, sometimes, you don't really know a person until after
marriage. If differences weren't already obvious pre-marriage,
it'll definitely become glaringly obvious after.
I guess one just have to be careful about making the right choices.
Then again, I'm sure that if you asked those who are
divorced, at the time of marriage, they thought that they had made
the 'right' choice, so sigh... it's just not easy... 
