I think because of problems like economics, lack of love or job, infidelity, etc...The most of couples have problems but this always lead to breaking.
Because it's more acceptable now. Some marriages just deteriorate over time. It's a good thing for both partners that they can get out in that case.
My theory is that relationships or marriages break down at a constant rate (more or less at the same percentage, say 75%) throughout the ages, human nature being what it is. It is just that whereas in olden times (under Queen Victoria, for example) people stayed in the same household and in the same marriage even after the marriage had broken down, nowadays people just get a divorce and live apart.
The old notion was that one could live independently from one's legally wedded spouse without getting a divorce, whereas in these new puritanical times, that old practice is regarded as hypocritical and immoral.
People evolve and they can hardly predict how they would evolve.
Moreover nature does not care much about a lasting happiness between male and female,
nature only cares that their sprout survive and reach sexual maturity (this is in a very short
sentence what has been called 'sexual driven evolution' by scientists).
Marriage is of human origin, it is good, but it is not a natural thing, so what chances
have it to compete with other things that strictly pertain to Nature, like life.
For me a marriage is good enough if:
a) does not produce great sufference.
b) If there are sons they receive attention, love, education and a god start towards life,
all these things in substantial amount.
c) it leaves intact the identity of husband and wife,or make them evolve!
d) gives or gave serenity and moments of happiness
e) can eventually be broken withiut much bitterness when condition (a) is not met any more.
Anyway I hope - if ever - to find something more meaningful for myself and my endearing 'associate'.
I guess that these breaks nowadays may stem from the discrepancy in the couple's routine and schedule. Maybe the man or the woman needs to move out and they can't help being in a relationship due the distance and all the problems that come with it. Plus, being away from each other may demage the relationship beacuse wheather the person choose it or not, they meet different people and make new friends, which might become a negative point for the well-being regarding marriage.