I think there are many reasons why people drop out. but i think probably the most i have seen at my dojo is that people come and expect to take a couple classes and be unstopable but when they relize how long the hard proccess is they get discouraged.(those kind of people would probaby like a punch kick art such as karate, or kung fu better)i know when i desided to do aikido i made up my mind that i wasn't going to quit no matter what. i wasn't going to let any pain or discouraging feelings i had get in the way of my goal. but as an american i think our culture trys to find fast ways to the payoff of things. also disipline most people arn't used to and have a hard time with. i remember my sensi told me to rollfall while he went to help other students and he left me there for what seemed like and hour or so. and he also did something similar with me with the ongaurd position.
thinking back it doesn't seem that strenuous now but i remember wishing he would come back so i could stop. i think, how many people would have the disipline and respect to let a sensi mold them in this fasion?
anyway these are a couple of my thoughts about this.
sorry about my many spelling errors