(CNN) President Donald Trump's views on foreign policy -- and, really, everything -- are surprisingly simple: He likes people who are nice to him and do things he wants and he doesn't like people who aren't nice to him and don't do things he wants.

So, his views on any given issue or foreign leader are largely informed by how that person has treated Trump in their last interaction -- and how much of what he wants that they are willing to give him.

That worldview is important to keep in mind as Trump travels to Canada to attend the G7 summit on Friday, a trip which he has already tried to wiggle out of, is cutting a day short and, if Twitter is any indication, is assuming will be a total and complete disaster.

"The European Union treats us very unfairly," Trump said as he left the White House Friday morning to head to the G7. "Canada [treats us] very unfairly."

That's the latest in a series of increasingly frustrated and angry comments coming out of the White House over the past few days, as its become more and more clear that leaders Trump thought were his friends -- French President Emmanuel Macron and Canadian Prime Minister Justin Trudeau -- have pushed back on his demands, particularly on tariffs.