RIB METE:

Marriage helps physical, mental health (CBC News, 1/27/11)
It is probably worth it to make the effort to say "I do," as experts in Great Britain suggest in a review that, on average, married people live longer and men enjoy better physical health, while women enjoy better mental health.

The review, which looked at 148 studies and was published in the British Medical Journal Thursday, concludes that the reason men enjoy better health when married is their partner's positive influence on lifestyle. The mental health bonus for women, the researchers say, may be because women place great value on the importance of the relationship itself.

The most widely accepted explanation for marriage being a good thing is that being in a committed relationship means better social support is available. It starts with the spouse and expands to a network of supportive relationships from there, the study suggested.
It's not that complicated:

And the LORD God said, It is not good that the man should be alone; I will make him a help meet for him.

And out of the ground the LORD God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof.

And Adam gave names to all cattle, and to the fowl of the air, and to every beast of the field; but for Adam there was not found a help meet for him.

And the LORD God caused a deep sleep to fall upon Adam, and he slept; and he took one of his ribs, and closed up the flesh instead thereof.

And the rib, which the LORD God had taken from man, made he a woman, and brought her unto the man.

And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman,

Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.