The difficulty changes are designed to keep blocks being found at a steady pace.  So when the total hash rate of the network goes up, and blocks start getting found faster, the difficulty will go up so that blocks will continue to be found at the same rate.

That's how we can predict when the next block halving will be.  It's all about math.  And the reason your reward goes down, is because there is more hashing power finding about the same amount of blocks.  So if you don't increase your hash power at the same rate the network/pool hash rate increases, then your share of the pie will get smaller.