Heuristic and systematic processing
Behavior change is not mind control. You are not exerting some sort of evil super-power to enslave the users' minds. Instead, your solution should provide guidance and support. To be able to do that, we need to understand how our brain processes information from the outside world.
We have two modes to process information. Most of the time, we use heuristic processing, or at least we are closer to the heuristic processing end of the processing spectrum, which means we pretty much effortlessly and mindlessly choose. When we are in a good mood, this is more likely to happen. When the price is relatively low, or the action is easily undoable, we use heuristic processing. When the message is seemingly too complex, we also tend to use this processing mode. For example, when buying a PC laptop, if you are not tech savvy, you might be easily overwhelmed by different specifications.
In the heuristic processing mode, the user is easily influenced by attractiveness...