We're all going to die, yes. But it's going to happen to Hazel and Augustus much sooner. Death follows these kids wherever they go. Even as they go through the usual teenage activities like playing video games, flirting, and falling in love for the first time, they are majorly aware of their own mortality. It's bleak, but it sure adds another layer of gravity to The Fault in Our Stars.
Hazel is so fixated on her own mortality and how it will hurt others that she doesn't stop to consider that everyone close to her could die and hurt her, too.
Hazel and Augustus come back to the idea of infinities in An Imperial Affliction again and again because it makes it easier for them to deal with the fact that they have limited time.