Step Brothers

39-year-old Brennan Huff and 40-year-old Dale Doback are men-children still living at home. Brennan lives with his mother, Nancy; and Dale has been living with his father, Robert; the parents meet and fall in love. Robert and Nancy get married, forcing Brennan and Dale to live with each other as step brothers.
Brennan and Dale dislike each other, blaming the other for interfering with their lifestyles. Brennan plays Dale's drum set and a brawl on the front lawn ensues. They are grounded for a week and they must find jobs within a month or they will be forced out of the house.
When Brennan's younger brother Derek, a helicopter leasing agent, comes to visit with his family, he mocks their unsuccessful lives and being spoiled brats, which entices Dale to punch him in the face. Brennan is awed that Dale stood up to Derek, while Derek's wife Alice finds Dale's courage a turn on, beginning a sexual affair between the two. Brennan and Dale use the incident to discover their shared interests and develop a brotherly friendship.
Robert schedules several job interviews for Brennan and Dale, who offend the interviewers with their disrespectful attitudes. Robert and Nancy reveal their plans to retire and sail the world on Robert's boat and allow Derek to sell the house, and demand that Brennan and Dale attend therapy and find other living arrangements.
Dale and Brennan decide to start an entertainment company, "Prestige Worldwide". They present their first music video at Derek's birthday party, which includes footage of them accidentally wrecking Robert's boat. Robert refuses to make an investment and spanks Brennan after a heated verbal exchange between the two. On Christmas Day, Robert announces he wants to divorce Nancy, causing Brennan and Dale to break down.
Blaming each other for their parents' divorce, the boys fight, but they decide to go their separate ways. Brennan works for Derek's helicopter leasing firm and Dale works for a catering company. Weeks later, Brennan and Dale are fully mature adults and are becoming successful.
Brennan manages the Catalina Wine Mixer for Derek's company and invites Robert and Nancy. They hire the catering company for which Dale works. The party is a success, until the lead singer of Uptown Girl, a 1980s Billy Joel cover band, responds to a man in the crowd by using explicit profanity, and is removed from the event. With the stage empty, Derek fires Brennan for the singer's mishap, Robert ultimately encourages Brennan and Dale to be their eccentric child-at-heart selves again. Brennan and Dale take the stage and perform "Con Te Partiro." Derek is so moved by the performance that he and Brennan make amends. Dale terminates his affair with Alice to her dismay.
Six months later, Robert and Nancy are reunited and move back into their old home. Brennan and the therapist Denise get married, while Brennan and Dale form a successful entertainment company that runs karaoke events. Robert has his boat repaired and put in the backyard. In a mid-credits scene, Brennan and Dale stand up to a group of bullies from a middle school who abused Dale a year before. In a post-credits scene, Brennan and Dale sleepwalk once more.

Question: Why do the brothers dislike one another?
Answer: They blame the other for interfering with their lifestyles