What has fear got to do with anything? Quite a lot actually!
Fear is the underlying reason that we smoke. If you struggle to stop smoking cannabis, then it is quite probable that you are actually scared and fearful to stop smoking marijuana. The reason is because your subconscious mind feels anxious and worries when you do not smoke because it is feeling a fear and doesn't know where it has come from. It is mainly due to your mind creating the belief that without a smoke you will feel anxious, worried and cannot relax properly without it.
This is an unknown fear to your body and is probably the worst type of fear. This fear then starts to tell your body it needs it's regular smoke by giving you symptoms, thoughts and cravings.
Your unconscious mind is actually trying to protect you in the short term by trying to eradicate this fear that you have developed. Although it wants to protect you from long term dangers such as risks of smoking, it senses a more immediate danger in the now and does what is necessary to get you to smoke to prevent feeling fearful.