My understanding of it isn't that life isn't literally suffering, but that anything can cause you suffering (or pleasure).

Let's say you don't own a car, and take the bus to work. Then someone gives you a car, you are happy! A week later someone steals your car. You are back to riding the bus. Your first day back on the public transport is probably filled with quite a bit of suffering even though it is no different that the bus ride you took a week ago.

It's also possible to be too attached to joy for fear that if it ceases it will cause you pain. To goal is to be happy despite external events.