I think that has many reasons. For one the things you mentioned all usually don’t require much focus in the first place. Like unless you somehow work with the music, seriously try to learn something from youtube videos or try to analyze tv shows you’re usually just there ignoring most of what isn’t part of that satisfaction you’re after.
That’s the second point, studying, working out or doing some job usually require actual focus without bringing immediate satisfaction. So while watching a tv show provides some “value” (as in hormone response) without much work, actual work has quite little “return on investment”.
Thirdly it depends on your interests. I for instance have a hard time studying but at the same time I can easily work for hours on end, even in my private time, on projects related to my student job. Why? Well because the process of studying often involves a lot of work without any results while my job, software engineering, is fun to me and provides immediate results (even though not necessarily positive).
Dopamine. Dopamine is a chemical in the brain that’s part of the reward mechanism that’s evolved to entice us to do things. When we get a good grade, a job promotion or someone compliments us on how buff our bodies are, the brain gives itself a hit of dopamine. However, the actions themselves we take to get that good grade, promotion or hot bod don’t usually provide that dopamine hit.
YT, TV, games, FB, etc. are specifically designed to provide a hit as you use them. The same with sex, drugs, gambling, etc. These are generally called “immediate gratification” activities. That is when you do them, you get an immediate dopamine hit. The temptation to do an immediate gratification activity can be overwhelming over a delayed gratification activity.
Latest Answers