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.
You go out into the world and do your best. You have your ups and downs, your problems and triumphs, your fun and tribulations. You charm and you perform.
Then you come "home" to a friend. You can relax, put up your feet; you are relieved. If you still have to be charming and/or performing, it's not a relief.
Friendship is a comfy situation like home. You get home, kick off your shoes, relax and sigh, "Ahh, home."