Rocky V

Shortly after Rocky Balboa's victory over Ivan Drago in Moscow, he, his wife Adrian, his brother-in-law Paulie, and his trainer Tony "Duke" Evers return to the United States, where they are greeted by Rocky's son, Robert. At a press conference, boxing promoter George Washington Duke attempts to goad Rocky into fighting his boxer Union Cane for the World Heavyweight Championship in Tokyo, but Rocky declines the offer.
Shortly after returning home, it is discovered that Paulie unknowingly had Rocky sign a "power of attorney" over to Rocky's accountant, who had squandered all of his money on real estate deals gone sour; in addition, the accountant had failed to pay Rocky's taxes over the past six years, and the mansion is discovered to be unpaid by $400,000. His lawyer confirms this, but he tells Rocky the situation is easily fixable with a few more fights. Rocky contemplates on accepting Cane's challenge, but after discovering he has sustained permanent brain damage from the fight with Drago and at Adrian's insistence, Rocky retires from boxing. He files for bankruptcy, has his mansion and belongings auctioned off and moves his family back into Paulie's old house in South Philadelphia. Adrian returns to working part-time at the J&amp;M Tropical Fish pet shop while Paulie goes back to the Shamrock Meat Packing plant. Rocky makes plans to refurbish and reopen Mighty Mick's Boxing Gym, willed to his son Robert by his late trainer Mickey Goldmill (thus safe from the crooked accountant's hands). He walks through the dark, abandoned gym and reminisces about a training session between the two just before Rocky fought with Apollo Creed years before, where Mickey gave Rocky a cufflink given to him by Rocky Marciano.
One day, Rocky and Paulie meet a hungry young fighter from Oklahoma named Tommy Gunn, and Rocky takes him under his wing. Training the young fighter gives Rocky a sense of purpose, and Gunn fights his way up the ladder to become a top contender. Rocky eventually becomes so distracted with Gunn's training that he winds up neglecting Robert, who becomes withdrawn and angry. He falls in with the wrong crowd at school and as a result, he begins acting out at home. Meanwhile, Gunn's impressive rise through the ranks catches the eye of Duke, who uses the promise of a heavyweight title shot against the newly crowned champion Cane to lure him away from Rocky. Duke also exploits the fact that Rocky does not have any contractual obligation to manage Gunn.
On Christmas Eve, Duke visits the Balboa house with Gunn in tow, who has now been deceived into thinking that Rocky does not have his best interests in mind. When Rocky tries to talk him out of siding with Duke, Gunn drives off in a huff, leaving Rocky for good. Adrian attempts to comfort Rocky, but his frustrations finally boil over. He confesses his life had meaning again when he was able to live vicariously through Gunn's success. She reasons with him, telling him Tommy never had his heart and spiritâsomething he could never learn. When this realization hits him, Rocky embraces his wife and they begin to pick up the pieces. After finding Robert hanging out on a street corner, Rocky apologizes to his son and they mend their broken relationship.
Gunn wins the heavyweight title by knocking out Cane in the first round, but is booed by spectators and hounded by reporters after the fight. They insist Cane was nothing but a "paper champion", because Cane did not win the title from Balboa. Therefore, the public would never consider Gunn the real champion unless he fights a worthy opponent, like Rocky; they drive the point home when one reporter announces, "...he's no Rocky Balboa!" With Gunn incensed by the press's reaction, Duke convinces him that he needs to fight Rocky man to man. Duke and Gunn show up at the local bar to goad Rocky into accepting a fight. Rocky declines the challenge and tries to reason with him, but Gunn rebukes it and calls him weak, prompting Paulie to stand up for Rocky. However, Gunn punches Paulie and he falls to the ground. Enraged, Rocky accepts the challenge but tells Gunn "my ring is outside."
Despite Duke's warnings to keep the fight in the ring, Gunn accepts. During the fight, Rocky is eventually beaten down and is seemingly out for the count. He then hears the voice of Mickey urging him to get up and return in the fight, to go just "one more round". Rocky gets up and, utilizing his vast street fighting knowledge as Robert, Paulie and the crowd root for him, is able to knock out his former protÃ©gÃ©. While Gunn is escorted away by the police, Duke threatens to sue Rocky if he touches him, but after a brief hesitation and with nothing else to lose anyway, Rocky knocks him onto the hood of a car and quips, "Sue me for what?"
The next morning, Rocky and Robert take a jog to the Philadelphia Museum of Art, and Rocky gives his son Rocky Marciano's cufflink, given to him years ago as a gift from Mickey. The film ends with a shot of Rocky's statue looking out over the Philadelphia skyline.

Question: Why does Rocky get back up and keep fighting?