Wonderful things are especially wonderful the first time they happen, but their wonderfulness wanes with repetition. Just compare the first and last time your child said "Mama" or your partner said "I love you" and you'll know exactly what I mean. When we have an experience—hearing a particular sonata, making love with a particular person, watching the sun set from a particular window of a particular room—on successive occasions, we quickly begin to adapt to it, and the experience yields less pleasure each time. Psychologists call this habituation, economists call it declining marginal utility, and the rest of us call it marriage.
From Chapter 7 (excerpts) of Daniel Gilbert's Stumbling on Happiness.
This page lists ten 'intriguing' quotes from the book. (For example, here's No. 9: "To learn from experience, we must remember it, and for a variety of reasons, memory is a faithless friend.")