Sometimes people are just bad at predicting their own level of commitment or disentangling desires and reality. Or they know they are "supposed to" do something

Sometimes people are just bad at predicting their own level of commitment or disentangling desires and reality. Or they know they are "supposed to" do something so they promise it or even believe that this time it will work... This is often the case with diet and exercise and studying hard. Then sweets, sitting on the couch and procrastination happen. Then the cycle repeats. Also in some cultures it's rude to reject invitations so people always say yes but it's understood implicitly by both parties that it's just politeness. The US tends to be like this, always saying stuff like "you should come over for dinner sometime" etc and you must reply enthusiastically but both the invitation and the acceptance count only in reality if a date and time is attached. This is often baffling to continental Europeans for example who tend to be more direct and would follow up with a message next day, asking when they should come over. Which is awkward because it wasn't a real invitation.