To understand this, first you must understand the state of superhero films at the end of the 20th century. Christopher Nolan had not yet made 'Batman Begins.' The 'Avengers' series was just a twinkle in someone's eye. Superhero movies had a bad reputation, and the last few attempts to take them to the big screen had failed. Val Kilmer's career was effectively ruined by playing Batman. Actors were a little wary of taking on a part that they'd be heavily associated with, and then ridiculed for if it went wrong. Josh Hartnett, therefore, believed he was protecting his career when he passed on the chance to become Spiderman.
While we can understand that, what happened next is less understandable to us. Having turned down 'Spiderman,' he watched as the movie became a success. Then he was offered 'Batman,' passed again, and watched Christian Bale redefine the role. A superhero opportunity came knocking again with 'Superman,' but he allowed Brandon Routh to take that part instead. To be fair, he was probably right about the last one!