Rock Star

Chris Cole (Mark Wahlberg) is a Pittsburgh fanatical admirer of a 1980s heavy metal band called Steel Dragon. By day, Chris is a photocopier technician and by night, he is the lead singer of a Steel Dragon tribute band called Blood Pollution (the name is taken from a Steel Dragon song).
Internal struggles among the actual Steel Dragon band members culminate with the firing of their lead singer, Bobby Beers (Jason Flemyng), and the starting of recruitment sessions to find a new vocalist. Chris experiences his own strife with his Blood Pollution bandmates, particularly guitarist Rob Malcolm (Timothy Olyphant). During a live performance, Rob's playing fails to live up to Chris' over-demanding standards regarding note-for-note accuracy to the original Steel Dragon recordings, and Chris sabotages Rob's amplifier mid-song - a fight breaks out between the two onstage. The next day, Chris arrives at Blood Pollution's rehearsal space (the basement of an X-rated theater) to find that he's been fired and replaced with his arch-rival, the (now former) lead singer of another Steel Dragon tribute band. Rob also cites Chris' inability to create his own musical style, preferring to remain the singer in a tribute band.
One day in 1984, Chris receives an unexpected phone call from Steel Dragon's founder and rhythm guitarist, Kirk Cuddy (Dominic West), and is offered an audition for the band (thanks to two of Blood Pollution's groupies, who showed Kirk a videotape of one of Blood Pollution's concerts). After hanging up on Kirk once, thinking he's being made fun of, Chris ecstatically agrees. At the studio, he meets the band, as well as learning that Bobby Beers was fired because he was a closeted gay, and gives an outstanding performance of "We All Die Young" (a Steel Dragon song in the movie, but it is actually a song by Steelheart, whose lead vocalist Miljenko Matijevic provides Cole's singing voice for the film). Chris joins the band as their new singer, adopting the stage name "Izzy". Following a successful debut concert with Steel Dragon, Izzy must come to grips with the pressures of his new-found fame and success. The band embarks on a lengthy tour and Izzy experiences the excesses of the lifestyle, with the group's manager, Mats (Timothy Spall), serving as a sympathetic mentor to Izzy.
His new lifestyle impacts his life both for better and worse, particularly with his relationship with his supportive girlfriend, Emily Poule (Jennifer Aniston), when she decides not to continue with him throughout the remainder of the tour as a rock star girlfriend, though Emily and Izzy agree to get back together when the tour reaches Seattle. Eventually, Steel Dragon stops in Seattle for a show, and Emily arrives at his hotel room as they had previously arranged, although Izzy had become so inebriated while on tour he forgot about the arrangement and did not even know what city he was in. Although taken aback by all the groupies, Emily still tries to reconnect with him, reminding him of their plans to meet up once he got to Seattle, however he is too intoxicated to really understand what she is saying, eventually suggesting they go to Seattle together. Heartbroken with his inconsiderate behavior, intoxication and the fact that he is sleeping with so many groupies, Emily leaves him.
After the end of the tour, Izzy reports to the next series of Steel Dragon recording sessions with song concepts for the band's next album. The rest of the band rejects Izzy's ideas, with Kirk explaining that the band has to stay true to the "Steel Dragon thing" to fulfill fan expectations. Izzy is angered upon realizing that he was only recruited for his vocal abilities. After a heartfelt conversation with Mats about how he feared he had no control over the direction life has taken him, Izzy begins to reconsider his rock star lifestyle. On the next tour, in a scene directly paralleling one near the beginning of the film with their roles reversed, Izzy hears a fan (Myles Kennedy) singing along with him toward the end of a live concert. Impressed, Izzy pulls the fan, who introduces himself as Thor, onstage and hands him the microphone to finish the concert. Backstage, Izzy realizes that what he wanted for so long was not what he thought, and he says goodbye to Mats, departing from the band while doing so.
Upon ditching his stage name, Izzy, Chris makes his way to Seattle and starts a new band with his old friend and former bandmate Rob. At the same time Steel Dragon, failing to evolve to changing tastes and styles, has its fame fizzle out. Chris finds Emily working in the coffee shop she and her roommate purchased a few years earlier, but is initially too ashamed to speak to her. While walking one evening, Emily sees a flyer for his band posted on the wall and takes it down. In the final scene, Chris is singing with his band in a bar and Emily walks in. Chris leaves the stage and speaks to her. They reconcile, ending the film with a kiss and the final note of Chris' first original song "Colorful" (which is actually a song by The Verve Pipe).

Question: Why does Emily leave Izzy?