Asked about Canada's retaliatory tariffs, Sarah Sanders says, "We've been very nice to Canada for many years, and they've taken advantage of that. Particularly advantage of our farmers."
Again: Canada does have some significant agricultural trade barriers, notably with regard to dairy and poultry, but it has regularly been the number-one export market for U.S. farmers, including last year. USDA data: 
Sanders added: "Escalating tariffs against the United Sates does nothing to help Canada and it only hurts American workers. The president is working to fix the broken system, and he's going to continue pushing for that."
Broader context: U.S. farmers are, on the whole, strongly supportive of NAFTA. Per the American Farm Bureau: "Agricultural exports from the U.S. to Canada and Mexico have increased from $8.9 billion in 1993 (the last pre-NAFTA year) to $38.1 billion in 2016."
Basically, what's happening is that Trump has seized on Canada's dairy protectionism as a useful way to beat Canada over the head on trade and NAFTA more broadly, though ag trade is generally quite free and though U.S. farmers generally do not agree that Canada is bad to them.
The U.S. has trade surpluses with more than half of all trading partners. Sanders, copying Trump's regular false claim, says, "We have been in trade deficits with nearly every country across the globe for years. And the president wants to ensure that that doesn’t continue.”
Asked if the U.S. is "winning" this trade battle given all the countries retaliating, Sanders says, “Again, the president is focused not on the short term but on the long term. And he wants to make sure that we’re doing things that help protect American workers..."
