taken from someone's blog - Don't let your choices depend on this article, but it's worth the read.

And yet, on rare occasions, I would see old couples who somehow seemed to
glow in each other's presence. They seemed really in love, not just
dependent upon each other and tolerant of each other's foibles. It was an
astounding sight, and it seemed impossible. How, I asked myself, can they
have survived so many years of sameness, so much irritation at the others
habits? What keeps love alive in them, when most of us seem unable to even
stay together, much less love each other?

The central secret seems to be in choosing well. There is something to the
claim of fundamental compatibility. Good people can create a bad
relationship, even though they both dearly want the relationship to succeed.
It is important to find someone with whom you can create a good relationship
from the outset. Unfortunately, it is hard to see clearly in the early
stages. Sexual hunger draws you to each other and colors the way you see
yourselves together. It blinds you to the thousands of little things by
which relationships eventually survive or fail. You need to find a way to
see beyond this initial overwhelming sexual fascination. Some people
choose to involve themselves sexually and ride out the most heated period
of sexual attraction in order to see what is on the other side.

This can work, but it can also leave a trail of wounded hearts. Others deny
the sexual side altogether in an attempt to get to know each other apart
from their sexuality. But they cannot see clearly, because the presence of
unfulfilled sexual desire looms so large that it keeps them from having any
normal perception of what life would be like together.

The truly lucky people are the ones who manage to become long-time friends
before they realize they are attracted to each other. They get to know each
other's laughs, passions, sadness, and fears. They see each other at their
worst and at their best. They share time together before they get swept up
into the entangling intimacy of their sexuality.

This is the ideal, but not often possible. If you fall under the spell of
your sexual attraction immediately, you need to look beyond it for other
keys to compatibility. One of these is laughter. Laughter tells you how much
you will enjoy each others company over the long term.

If your laughter together is good and healthy and not at the expense of
others, then you have a healthy relationship to the world. Laughter is the
child of surprise. If you can make each other laugh, you can always surprise
each other. And if you can always surprise each other, you can always keep
the world around you new.

Beware of a relationship in which there is no laughter. Even the most
intimate relationships based only on seriousness have a tendency to turn
sour. Over time, sharing a common serious viewpoint on the world tends to
turn you against those who do not share the same viewpoint, and your
relationship can become based on being critical together.

After laughter, look for a partner who deals with the world in a way you
respect. When two people first get together, they tend to see their
relationship as existing only in the space between the two of them. They
find each other endlessly fascinating, and the overwhelming power of the
emotions they are sharing obscures the outside world. As the relationship
ages and grows, the outside world becomes important again. If your partner
treats people or circumstances in a way you can't accept, you will
inevitably come to grief. Look at the way she cares for others and deals
with the daily affairs of life. If that makes you love her more, your love
will grow. If it does not, be careful. If you do not respect the way you
each deal with the world around you, eventually the two of you will not
respect each other.

Look also at how your partner confronts the mysteries of life. We live on
the cusp of poetry and practicality, and the real life of the heart resides
in the poetic. If one of you is deeply affected by the mystery of the
unseen in life and relationships, while the other is drawn only to the
literal and the practical, you must take care that the distance does not
become an unbridgeable gap that leaves you each feeling isolated and
misunderstood.

There are many other keys, but you must find them by yourself. We all have
unchangeable parts of our hearts that we will not betray and private
commitments to a vision of life that we will not deny.

If you fall in love with someone who cannot nourish those inviolable parts
of you, or if you cannot nourish them in her, you will find yourselves
growing further apart until you live in separate worlds where you share the
business of life, but never touch each other where the heart lives and
dreams. From there it is only a small leap to the cataloging of petty
hurts and daily failures that leaves so many couples bitter and unsatisfied
with their mates.

So choose carefully and well. If you do, you will have chosen a partner with
whom you can grow, and then the real miracle of marriage can take place in
your hearts.
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