US and UK has had what some call a "special relation" for decades. Just a glimpse of recent declarations about this subject (Wikipedia):
In 2015, Cameron stated the US President calls him "bro" and described the "special relationship" between Washington and Westminster as "stronger than it has ever been". In March 2016, the US President criticised the British PM for becoming "distracted" over the intervention in Libya, a criticism that was also aimed at the French President. A National Security Council spokesman sent an unsolicited email to the BBC limiting the damage done by stating that "Prime Minister David Cameron has been as close a partner as the president has had."
However, there has been some tension building up lately between the two countries. Classified police photos from the scene of the terrorist attack in Manchester, England, leaking to The New York Times is one example:
British security officials reacted with fury to the leaks, with one telling the Financial Times that lives were being put at risk. The BBC also reported on Thursday morning that the police in Manchester had stopped sharing information about the investigation with the US because of the ongoing leaks.
Following the terrorist attacks in London at the beginning of June, Donald Trump made a remark that many would see as unfit:
President Trump has launched a scathing attack on Sadiq Khan, the mayor of London, over his reaction to the attacks in Borough Market and on London Bridge.
It is clear that one has reasons to be alarmed after a terrorist attack had place in his/her city, but this remark is not exactly nice after such an event.
Question: Why does Donald Trump seem to help raising of the tension between US and UK ?