There are many themes and messages in the novel The Sun is Also a Star, and it is easy to boil it down to the idea that fate plays a role in our lives and that we must be willing to follow destiny. However, I believe that there is a deeper message, because the events that occur in the novel do not necessarily seem predestined or fateful all the time.
In fact, only the return of Daniel and Natasha to the same coffee shop seems to follow the idea of destiny in fate. Instead, the central message seems to be the idea that suffering and pain, in whatever form they take, can lead to beautiful things. For instance, even though Jeremy is injured by a car, he eventually falls in love with and marries the doctor who attended to him. And while Daniel is heartbroken over Natasha's departure, he pursues poetry and attends a different college as a result, which leads him to a happier life. Natasha, also, having been deported, is able to eventually be a top student and get a graduate student visa to return to New York, where she is reunited with Daniel.
It all echoes the scientific overture at the beginning—in relation to the Big Bang. The cosmic act of apparent destruction and chaos brought about beauty and life itself as it coalesced into the universe we know today.