Countries are pretty large organizations, however, governing principles of relationship between countries are similar to any other social group. How do you know that any of your agreements are going to be honored?
I will give few examples:
- How do you know your friends will show up to your birthday party? They did in the past 10 years, and you talk every week, and you didn't hurt them too badly recently. All these factors give you good chance they'll show up
- How do you know that when you bring lettuce to the check out, the price will be exactly as it was advertised in the veggie isle? Well, the store is pretty large, and you had good experience before. Also you know that if they try to defraud anyone, they might get backlash from social media. Plus, possible benefit the store can get from this fraud is minuscule compared to risk. Plus, you can always leave and go to any of many supermarkets around.
- How do you know the bank will not steal your money? There are laws and pretty stringent enforcing agencies, apart from everything else I said about grocery store.
Between countries, of course, relationships are harder. But I believe, you can use normal life analogies to understand some of the recent events.
Britain wants to get out of European Union (doesn't want to show up to party). That is because UK thinks EU stinks as a friend, for example, because they allegedly didn't listen to them for years.
Trump decided to engage in trade war and increase tariffs on good exchanged with China. Compare that to grocery store which decided to crank up price of potato overnight, because new management feels the price was unfair. It might work for store's benefit, but only if there is no other grocery store around, and market is cornered. In US-China situation, there is no another easily accessible market for both of them, so they have to lose money by either paying higher tariffs or establishing new ways of selling goods (trade with other countries).