Well, not using version 3 or El Capitan i am guessing, but the usual reason is:
Connect your camera
In finder go Applications > Image Capture.app
Down the bottom there will be a funny little icon. Click it and it will ask you what yo do when you connect a camera. Tell it.
My opinion though is to leave this off and just copy from the card to the hard drive you use. Much faster, safer and more reliable.