Kristin Cavallari's desire to continue focusing on her entertainment career was reportedly the reason behind her ex-fiance Jay Cutler ending their relationship.  

According to Life & Style, Cutler broke up with the 24-year-old former The Hills star after the couple got into a July 22 argument about her refusal to leave Los Angeles and fly to Chicago with him the following day. 

"Kristin said she didn't want to because she's working on a new TV project and had a full day of shooting scheduled for Tuesday [July 26]," a source told Life & Style. "He told her that she needed to commit to him, and to move to Chicago."

It became apparent to the 28-year-old NFL quarterback that Cavallari had no intention to stop working, pick up her life and move to Chicago -- a decision that led Cutler to split from the former reality TV star and return to Chicago alone on July 23, the magazine reported.

The breakup reportedly came as a complete shock to Cavallari, who had been gushing about her marriage plans during a wedding dress photo shoot with Life & Style only a day before the big fight. 

"I want a romantic, mystical wedding, with lots of flowers," Cavallari -- who also appeared on Laguna Beach, MTV's The Hills predecessor, while she was in high school -- told the magazine during the shoot at LA's Mondrian Hotel on July 21.

However, the couple -- who began dating last August after they met through Giuliana & Bill star and E! News host Giuliana Rancic and then got engaged in April when Cutler proposed during a spontaneous vacation to Cabo San Lucas -- reportedly had other problems as well, as Cavallari wasn't ready to have children yet.

"Jay's family was pushing Kristin to get pregnant immediately. Kristin knows she's too young for kids right now," the source said, adding that Cavallari "really loved Jay" and is "devastated" over the breakup.