What keeps the average block time at 10 minutes?
The bitcoin network rules define which difficulty each block has. This is done through a simple formula that only depends on the block chain itself. This means that if you give me a blockchain with blocks 1 through N, I can tell you with 100% accuracy what the difficulty of block N+1 will need to be, and I can reject any block which has the wrong difficulty. This means it is not really "the network" deciding on a difficulty, but really each separate node calculating and enforcing it independently.

The actual formula is this: at every block N which is a multiple of 2016, look at the time stamps of the past 2015 blocks, and change the difficulty for what follows to old_difficulty*(2 weeks)/(time the past 2015 blocks took). Additional rules on which time stamps are allowed in blocks prevent cheating (mostly).

In short, by using a bitcoin client, you agree to its network rules, and those include the difficulty adjustment.