Hello!
The title comes from a line in one of Shakespear's plays.
"The Fault in Our Stars" is kind of like another way to say "our really bad luck". I believe the title relates to events that take place in the story. Events such as Hazel getting cancer, Augustus getting cancer and having his leg amputated, and Gus' death.
Of course, Hazel and Gus didn't cause the cancer - it just happened. Although they went to the doctor and for treatments frequently, they weren't able to avoid it. Nor were they able to avoid Gus' death.