The problem is that happiness in a marriage varies heavily over time, and has lots of ups-and-downs.
I think a big thing that commitment devices (religion, family ...) help with is that when the going gets touch for a little while, you stay together, which is usually good in the long run.
If your marriage sucks for several years, or if there's abuse, then breaking it may be OK; but many of the issues you discard would also signal people not committed, and marriages breaking because they suck for a couple of months.
Yes, I agree that commitment has value. But the standard view promulgated my many religions (and expressed in some of the comments here) that marriage should be almost non-negotiably for life takes this WAY too far. I don't know where the balance is, but I am positive that it is nowhere near where tradition places it.
> But the standard view promulgated my many religions (and expressed in some of the comments here) that marriage should be almost non-negotiably for life takes this WAY too far.
It depends on what you think is the purpose of marriage. The traditional view you're questioning is that marriage is a very serious contract that is never to be broken (usually with a few exceptions, like infidelity or abuse). I suspect this view is historically an economic device for raising children: the woman raises children while the man provides for the family (please note, I'm making no normative argument for these roles). Monogamy is obviously historically useful for ensuring paternity, and there's obvious selective pressure for that norm to arise (although I've seen studies claiming that humans have always tended to practice punctuated monogamy).
These days, that view of marriage is less vital in the Western world (at least the economic part), but it still has a lot of cultural momentum. Aside from that, the only modern purpose of official marriage is legal benefits: taxes, wills, medical discretion, child custody, etc. Apart from those, I see little difference in a "marriage" and simple long-term cohabitation other than the notion that the former is a life-long commitment.
On the "cultural momentum" of marriage, it varies a lot from one region to the other. For example in Canada, between Ontario and Quebec (in Canada), these 2009 stats are pretty interesting:
http://www.statcan.gc.ca/pub/84f0210x/2009000/t006-eng.htm
In short, in Quebec today, less than half of kids born are from non-married parents. Marriage is seen by many as irrelevant, bureaucratic, peer-pressure consumerist obligation. According to QC civil law, once a couple has a kid, they have the same legal obligations to the kid as a married couple (i.e. contributing to supporting the kid in a way proportional to their revenue/salary). As the tacky government slogans like to remind us.. "in a couple one day, parents always" i.e. marriage has little relevance.
I think a big thing that commitment devices (religion, family ...) help with is that when the going gets touch for a little while, you stay together, which is usually good in the long run.
If your marriage sucks for several years, or if there's abuse, then breaking it may be OK; but many of the issues you discard would also signal people not committed, and marriages breaking because they suck for a couple of months.