A good thesis statement for The Fault in Our Stars is that though life may be transient, it is nonetheless possible to deepen our experience of it by forming close personal relationships.
Life appears fleeting for many of us, but especially for those such as Hazel, who's terminally ill. And yet, despite her condition and despite her initial skepticism at forming relationships, Hazel meets someone, Augustus, who will give what remains of her life meaning and depth.
So many of us tend to look at life in quantitative terms, in the number of years we spend upon this earth. But thanks largely to her close loving friendship with Augustus, Hazel is able to gain a different perspective on life; to treat life in qualitative terms, and to get as much out of it as is humanly possible.
Hazel learns that what truly matters in life is the quality of the relationships that we forge with others, not how long we live. Of course, Hazel would like to live much longer, and her untimely death is as much a tragedy as Augustus's. But in the brief time she spends on this earth, Hazel is able to enrich her life and the lives of others by reaching out and seizing the opportunity to give what little time she has left meaning and purpose.