Trading card games, like the one in Steam, are social training and highly symbolic on a spiritual and philosophical level.

A trading card game is like a metaphor for life:

• Due to statistical chance, it is highly unlikely that any of us is given a full set of cards.
  
• We have cards of many sets.
  
• Fulfillment comes from succeeding at achieving the goal of the game.
  
• So in order to succeed, we need to complete at least some of our sets.
  
• Completing a set is impossible without other people's cards.
  

The world is full of people who are too much in the grip of an irrational fear that they could lose something (even if they're not aware of this root cause for their decisions) and thus do not contribute to the success of the playerbase as a whole. This, at best, causes success to merely be redistributed, but more likely for the afraid to not succeed either.

This leads to less cards in the game, lower chances of finding cards to complete sets. Everybody loses. ... Even those who try to steal cards from others, since they will eventually, after they cannot fool themselves anymore, experience the profound feeling of emptiness that comes from confusing means and ends of the game. And those who encounter enough of this type might themselves eventually become what they resent, and generalize so much that they become deaf to the solution knocking on the door.

Along these lines, my recent experiences with the Steam trading card game as well as many earlier experiences with people in general made me formulate this:

You do not use friendship to play a game.

You play a game to live friendship.