Friendship

The holy passion of friendship is so sweet and steady and loyal and enduring in nature that it will last through a whole lifetime, if not asked to lend money.
Be courteous to all but intimate with few, and let those few be well tried before you give them your confidence; true friendship is a plant of slow growth, and must undergo and withstand the shocks of adversity before it is entitled to appellation.

To ensure that the self doesn't shrink, to see that it holds on to its volume, memories have to be watered like potted flowers, and the watering calls for regular contact with the witnesses of the past, that is to say, with friends.
If you can survive 11 days in cramped quarters with a friend and come out laughing, your friendship is the real deal.