One factor that irritated Donald Trump ahead of his meeting with Volodymyr Zelensky was that the Ukrainian president arrived not wearing a suit. 

Source: Axios reported two sources from Trump's circle, "European Truth"

Details: Trump's advisors had repeatedly told Zelensky's team that it would be more respectful for the Ukrainian president to avoid military-style clothing during a visit to the White House.

Zelensky came to the White House in a more formal black outfit featuring the national emblem of Ukraine, but without a suit.

"He dressed up a bit today", – noted Trump as he shook hands with Zelensky at the entrance of the West Wing of the White House.

Volodymyr Zelensky has not worn a suit since the full-scale invasion of Russia in Ukraine. At one point, Polish President Andrzej Duda expressed a hope that Zelensky would attend a summit "in a suit", symbolizing the end of the war in Ukraine.

In the Oval Office on Friday, one reporter asked Zelensky why he did not wear a suit for the meeting in the "highest level" office of the United States.

"Why aren't you wearing a suit? Do you have a suit?" – asked the journalist.

"Do you have a problem with that?" – Zelensky replied.

"Many Americans take issue with those who don't respect the Oval Office dress code", – the journalist responded.

Following this, Zelensky stated that he would wear a suit after the Russian-Ukrainian war ends.