The Sisterhood of the Traveling Pants by Ann Brashares

Some friends just fit together.
 
Once there was a pair of pants. Just an ordinary pair of jeans. But these pants, the Traveling Pants, went on to do great things. This is the story of the four friends—Lena, Tibby, Bridget, and Carmen—who made it possible.
 
Pants = love. Love your pals. Love yourself.