The book of Tarquin's Love.
Chapter 1:  You fall in love, however it happens.  It's bliss.
Chapter 2:  You get married and/or shack up.  Your space is not your own, your
personality get compromised to suit her...but the sex is better.
Chapter 3:  Kids.  Ah, the cute bundles of joy.  Now, you are too tired for
sex, and any extra income you had is no longer extra.  Financial obligations
add stress to the relationships.
Chapter 4:  Watching the kids grow up.  You guide them, you teach them, you
laugh at their folly, and you share in their pains.  They take over your life
and you hardly even notice.
Chapter 5:  The kids move out.  Suddenly, you are staring across the table,
looking at this being that you have lived with for 25 years, and you think
'Geez, who the hell are you?'.
Chapter 6:  The End.  After realizing the goal of perpetuating the family
lines, the relationship has no purpose...drift away.