The other day, a White House reporter asked the president if Michael Cohen was, as President Trump put it, “a weak person and not a very smart person,” then why had he hired him? “Because a long time ago, he did me a favor,” Mr. Trump replied.

Mr. Trump didn’t say what the favor entailed, but a pair of Mr. Cohen’s sentencing memos released by federal prosecutors late last week reveal that the door to Mr. Trump’s office opened in 2007. The condo board at Trump World Tower in Manhattan, where Mr. Cohen lived, tried to remove the Trump name from the building. Mr. Cohen intervened, got the entire board thrown out, and was soon earning a $500,000 salary at Trump Tower.

One of the keys to success in Mr. Trump’s life has been collecting deposits in a bank of favors. “Do me a favor” is one of his favorite lines, with the promise of good things to follow. For some, the good things never materialize. But failure to play this game can land you a spot on Mr. Trump’s enemies list. For example, Mr. Trump nursed a long-held grudge against Mario Cuomo , because the former New York governor failed to grant him an unspecified “perfectly legal and appropriate favor.”

The problem for Mr. Trump is that he is unable or unwilling to spot the difference between a favor and a crooked scheme. And that goes a long way toward explaining why he has surrounded himself with people in trouble with the law and why his presidency is in the grip of intensifying federal investigations.