There are several important themes. One is the idea of finding beauty and meaning in a difficult life. Hazel and Augustus find joy in each other and in small things like books and shared experiences. Love is a central theme, not just romantic love but also the love between friends and family. The theme of mortality is also prominent. Their cancers constantly remind them of their limited time, and they have to come to terms with the idea of death while still trying to live fully.
The Fault in Our Stars has elements that are based on real - life situations. Author John Green was inspired by his time working in a hospital with young cancer patients. He incorporated their stories, emotions, and the unique challenges they face into the book. For example, the main characters' experiences with illness, their relationships with family and friends, and their search for meaning in the face of mortality are all things that real young cancer patients might go through.
One possible fault could be that the character development wasn't as deep as it could have been. It felt a bit rushed at times, leaving you wanting more depth.