It is an in-depth relationship combining trust,support,communication,loyalty,understanding,empathy,and intimacy.
These are certainly aspects of life that all of us crave.
Being able to trust and relax with your friend is a big part of friendship.
Remember when you were young and went with a friend to her grandma's for the week-end.It was fun but when you got home,home was wonderful.Your feeling was "I'm home.I can relax now."
That's what a friendship should be.