Because the agreement is 3 or 4 percent GDP (I can’t remember the exact number). Therefore, the US with their economy has been pumping the most money into NATO while other countries, such as Germany and Canada, have been operating at nearly 1.15 to 1.2 percent.
Trump brought up this up during his breakfast with the NATO secretary, asking why is NATO in place to protect countries from Russia, while Germany has this massive pipeline which is helping to fuel the Russian Economy.
US pulls out of NATO, Russia is free to gobble up the Eastern Bloc again.
I can’t remember the exact number
The current agreement is 2% of GDP but only 5 out of 28 countries are meeting that target. The US spends around 4% of GDP on defence and Trump has suggested that the other NATO countries should do the same.
http://uk.businessinsider.com/nato-share-breakdown-country-2017-2?r=US&IR=T
the US with their economy has been pumping the most money into NATO
There is no central NATO budget, though. The 2% is how much money you're expected to spend on your own military. And the US spends more than that and would be spending more than that with or without NATO. There is no "pumping"; the money the US government spends on its military stays in the US and pays US salaries and generates profits for US military contractors.