The wallet probably determined that the transaction may be safely sent without a fee because of these conditions being met:

• It is smaller than 10,000 bytes.
• All outputs are 0.01 BTC or larger.
• Its priority is large enough

Quick send automatically chooses the details for the transaction. If you want more control over your funds then use the "Custom" transaction type which allows you to set your own miners fee for the transaction so it may be included in a block quicker.