Small-town New Providence, New Jersey, attorney Mike Flaherty (Paul Giamatti) moonlights as a wrestling coach and struggles to keep his practice solvent, while shielding his wife Jackie (Amy Ryan) and their two young girls, Abby and Stella, from the extent of the problem. When his court-appointed client, Leo Poplar (Burt Young), who is suffering from early dementia, turns out to have no locatable relatives, he persuades a judge to appoint him as guardian, for which he will receive a stipend of $1,508 per month. Mike, however, has no intention of taking care of Leo and moves him to a senior care facility while he continues to get paid for guardianship.
When Leo's troubled teenage grandson, Kyle (Alex Shaffer) shows up from Columbus, Ohio, looking to live with him, Mike and Jackie let him stay with them instead. Kyle tries to break into Leo's old house, and when Mike and Jackie question him about it, he reveals his troubled family life: His mom is in rehab, she lives with her boyfriend, and he doesn't want to go back. Upon hearing this, Jackie refuses to allow Kyle to return home and lets him stay in their household. After Kyle sits in on practice, they discover that he is a talented wrestler and enroll him at Mike's high school, where he can resume his education and wrestle on Mike's losing team, helping to make them viable contenders in their league.
This "everyone benefits" setup is disrupted when Kyle's mother Cindy (Melanie Lynskey) shows up, fresh out of rehab. Cindy attempts to gain custody of her father and her son, and with them her father's substantial estate. However, Mike explains to Cindy and her lawyer that Leo had disinherited her from his will, causing her to become furious. Later, Cindy calls Kyle to her hotel room to show him court documents proving that Mike is supposed to keep Leo at home and not at the elderly home. Kyle reacts violently towards his mother before running away.
Upon learning the truth about Mike, the boy rejects him as a money-seeking opportunist no better than his mother. Realizing the mistake of his earlier actions, and seeking instead to do what's best for both Leo and Kyle, Mike offers Cindy the monthly stipend in exchange for leaving them in his care. He and Jackie take Kyle into their home permanently and return Leo to his, with Mike instead taking a job as a bartender to address his financial problems.

Question: Why does Mike become Leo's guardian?
Answer: Mike is having financial issues.
Alternative answer: Because he has dementia.