The Break-Up

Gary Grobowski (Vince Vaughn) and Brooke Meyers (Jennifer Aniston) meet at Wrigley Field during a Chicago Cubs baseball game and begin dating, eventually buying a condominium together. Gary works as a tour guide in a family business with his brothers, Lupus (Cole Hauser) and Dennis (Vincent D'Onofrio). Brooke manages an art gallery owned by eccentric artist Marilyn Dean (Judy Davis). They're pushed to the breaking point after their latest, "Why can't you do this one little thing for me?" argument. Brooke, feeling unappreciated, criticizes Gary's immaturity and unwillingness to work on their relationship. Gary, on the other hand, is frustrated by Brookes controlling, perfectionist attitude and expresses his desire to have more space, particularly after coming home from work. Brooke becomes upset when Gary fails to offer to help her clean up after a dinner party and, still frustrated from their earlier argument, breaks up with him. Brooke seeks relationship advice from her friend Addie (Joey Lauren Adams), while Gary goes to unload his feelings with the support of his friend Johnny Ostrofski (Jon Favreau).
Since neither is willing to move out of the condo, they compromise by living as roommates and both begin acting out to provoke the other. Gary buys a pool table, litters the condo with food and trash, and even has a strip poker party with Lupus and a few women. Meanwhile, Brooke has Gary kicked off their "couples only" bowling team and starts dating other men in an attempt to make Gary jealous. When their friend and realtor Mark Riggleman (Jason Bateman) sells the condo, Gary and Brooke are given a two week notice to move out. Brooke invites Gary to an Old 97's concert hoping he'll intuit the gesture as her last ditch attempt to salvage their relationship. Gary agrees to meet her there but fails to see the significance and misses the concert, which breaks Brooke's heart.
Afterwards, Brooke quits her job in order to spend time traveling Europe. When she brings a date home one evening, Brooke finds the condo cleaned and Gary preparing a fancy dinner in order to win her back. He lays his heart on the line and promises to appreciate her more. Brooke begins crying and states that she just can't give anymore and, therefore, does not feel the same way. Gary seems to understand and kisses her before walking out. It's later revealed that Brooke's "date" (who initially asked her out and was politely rejected) was actually a client interested in a piece of artwork she owned in her condo. When Gary goes out for a drink with Johnny, his friend points out the fact that Gary has always had his guard up, and never gave Brooke a chance because that has created such selfishness on his part.
They move out and Gary begins taking a more active role in his tour guide business while Brooke travels the world, eventually returning to Chicago. Some time later, they meet again by chance on the street as Gary is bringing home groceries and Brooke is on her way to a meeting. After some awkward but friendly catching up, they part ways but each glances back over their shoulder and they share a smile.

Question: why does the roommates act out?
Answer: to provoke the other to move