The Reward System
The brain has to learn to use a substance or an activity to stimulate the reward system. Once the brain makes the connection between an addictive substance or activity and feeling good, an addiction is potentially established. The addiction is formed in the subconscious brain; the user has no conscious control over it. It can be further reinforced by repeated consumption of the substance or carrying out of the activity. Compulsive behavior stemming from an addiction that is akin to a habit is ingrained on a subconscious level.
The brain seems to make a permanent connection with the object of addiction, especially when it comes to substances. Even if an individual were to break his addiction to a substance for some time, use of the substance in subject is likely to trigger a full blown addiction again.
Substance-addiction
Nicotine
Apart from the chemical aspects of nicotine addiction, there exist the psychological aspects to nicotine addiction as well. Smokers are likely to welcome smoking breaks during work, and socializing with other smokers when smoking. Such positive associations with smoking would create and compound psychological addiction to nicotine.
Alcohol
Activity-addiction
Gambling
Every bet a gambler makes has the potential for a pay-out, as well as a loss. A bigger bet brings about a bigger sense of thrill and gets adrenaline pumping. This would compound the gambling addiction further.
Other Activity-addictions
Certain events in video games, such as leveling up, gaining new skills for your in-game characters, or beating difficult levels or opponents, encourage players positively, and bring about a feeling of achievement. Some players would feel compelled to seek these empowering feelings of achievement again. Some mobile games offer timed events in which players stand to gain better or unique in-game prizes if they were to play harder and more often. This causes players to play the games like they were scheduled jobs.
Some Facebook addicts capture and share their lives' moments online excessively, while some scroll through Facebook feeds mindlessly and compulsively. The former ones have probably developed a self-imposed responsibility upon themselves to keep their online profile updated constantly. Some would do it in anticipation of positive comments or "likes", while some might be doing it because they cannot shake off the self-imposed chore they started due to obsessive compulsive tendencies. The latter ones are probably used to getting regular mental stimulation from their phones, and can no longer handle some down-time of having nothing to occupy themselves, and thus, reach for their phones and scroll through Facebook to relief themselves of idleness.
Ending of Addictions
Addictions can never truly be broken, as the brain is always capable of re-establishing them. However, they can be weakened, as long as we stop consuming the addictive substance or performing the addictive activity for a period of time. It is difficult to see when our point of addiction is at its strongest, but our brains would stop remembering and triggering the addiction so much, after a period of abstinence, if we allow ourselves this period. The longer this period goes on, the weaker the addiction becomes, and so, the triggers lessen and eventually disappear.
One strategy we can employ to bring forth the period of abstinence is by responding to triggers of addictions with positive habits or thought patterns. For instance, when tempted to scroll through Facebook or fire up a mobile game on the phone in a bored moment, welcome the opportunity to allow your mind to take a break, take deep breaths, and relax. It would take more effort to abstain from addictive substances due to their chemical effects on the brain. When receiving a trigger to smoke a cigarette or drink alcohol, one has to respond to it by remembering the fact that if he chooses not to indulge in the addiction at that point of time, it will only get easier as time draws out, as the memory of the addiction and the frequency of the triggers will be diminished. One can also form a positive obsession about being healthy, by eating healthy, natural foods and exercising regularly. This will steer one away from unhealthy substance-addiction.
When a long period of abstinence has passed, the addiction will lie dormant. However, in some situations, one might be reminded of the rewards they experienced before upon witnessing other people indulging in his former addiction, and be tempted to indulge in it, even if it is just once. Doing so holds a high risk of re-establishing the addiction, which will get full-blown eventually. Once an addiction is arrested and lying dormant, do not entertain the thought of indulging in it ever again, for doing so will most likely cause it to wake up in full force eventually.