Charlie (Harvey Keitel) is a young Italian-American man who is trying to move up in the local New York Mafia but is hampered by his feeling of responsibility towards his reckless younger friend Johnny Boy (Robert De Niro), a small-time gambler who owes money to many loan sharks. Charlie works for his uncle Giovanni (Cesare Danova), the local caporegime, mostly collecting debts. He is also having a secret affair with Johnny Boy's cousin Teresa (Amy Robinson), who has epilepsy and is ostracized because of her conditionâespecially by Charlie's uncle. Charlie's uncle, a dignified man who takes his role as caporegime seriously, also wants Charlie not to be such close friends with Johnny, saying "Honorable men go with honorable men."
Charlie is torn between his devout Catholicism and his Mafia ambitions. As the film progresses, Johnny becomes increasingly self-destructive and disrespectful of his creditors. Failing to receive redemption in the church, Charlie seeks it through sacrificing himself on Johnny's behalf.
At a bar, a local loan shark named Michael (Richard Romanus) comes looking for Johnny to "pay up", but to his surprise, Johnny insults him. Michael lunges at Johnny, who retaliates by pulling a gun on him. After a tense standoff, Michael walks away, and Charlie convinces Johnny that they should leave town for a brief period. Teresa insists on coming with them. Charlie borrows a car and they drive off, escaping the neighborhood without incident. But then a car that had been following them suddenly pulls up alongside, Michael at the wheel and his henchman, Jimmy Shorts (Martin Scorsese), in the backseat. Jimmy fires several shots at Charlie's car, hitting Johnny in the neck and Charlie in the hand, causing Charlie to crash the car. The film ends with an ambulance and police arriving at the scene, and paramedics take them away.

Question: Why is michael loking for Johnny?
Answer: To pay up.
Alternative answer: He works for the loan sharks and Johnny owes money