Sending less than 0.01 BTC to any recipient — The network considers these small outputs to be “dust,” and discourages them by requiring a fee. If it was not discouraged, someone could take 1.0 BTC, and create 1,000,000 transactions of 0.000001 BTC each, for free, which would clog the network.
Other reason, same outcome. Don't forget, if you move coins, even from your own wallet to another that you own, it still is included in the block chain and it uses resources of the miners. They just get a compensation.