13 Unspoken Rules of Gaming
You play to win.
Every game defines certain victory conditions, and the game only works when every player is pursuing those victory conditions. Essentially, the victory conditions are part of the rules, and should be followed just as strictly, otherwise it tends to ruin the fun for everyone else.
This second unspoken rule of gaming is, of course, subject to the first: if you have to play for some other goal other than winning in order for the game to be fun, then okay. But the other players need to also agree to this, or else you are just having fun at their expense.