A bit of trivia I find interesting: the etymology of the word passion is "to suffer (as christ did) [1]". I think in reality, DWYL advice is quixotic. But from within the head of an undeclared major, it sounds great. "Kid, just do whatever you like, and people will pay you for it!" And from the perspective of an employer, it sounds even better. "A passionate employee will put in 120% because he wants to do it anyway." So from both party's perspective, it's a win-win. I think this is primarily why DWYL is perpetuated.

[1] http://www.etymonline.com/index.php?term=passion&allowed_in_...