It's all about pain and pleasure. Whatever you focus on will be what you do, or dont do. If you focus on how good food tastes,your mind will make you want more of that pleasure of eatting that food and no amount of throwing up will stop you from eating. But if you focus on how bad you felt from throwing up, your mind will try to stop you from receiving that pain and then tell you that. Its the same with drugs/alcohol. If you focus on the high your are going to get, you are going to crave it again at another point and time. If you focus on the pychosis/paranoia or hangover you are going to get, your brain is not going to crave that stuff.