Marriage is a social contract negotiated between two people who generally have only a partially overlapping set of expectations. It is also symbolic and percieved by some as a mystical union, a bonding of souls (whatever a soul is). If it were regarded merely as a practical social device we might have more realistic expectations of the realities that continually pervade marriage, but the symbolic, intangible, mystical aspects continually invade our perceptions. I think our definition of marriage gets conflated with our confused conception of humanity and what it means to be human. Thus there is inevitably a religious element to our construction of what is in actuality nothing more than a convenient binding agreement between two people. You can make as many arguments as you like concerning the complex reasons for which particular people choose to be together on that level, but you can also reassess every one of those reasons through a filter of pragmatism with the same results.

Marriage is nice, and it's even nicer when you don't put unecessary strain on the relationship by thinking you've merged with your 'soul mate' and that two people become one.