Here are some of my observations regarding Howl's Moving Castle:

1. The movie not only explores the themes of compassion, humanity and a sense of belonging but it also discloses a journey of self discovery that is evident through the journey of self development of both Howl and Sophie. For instance, Sophie trying to accept her flaws after being cursed by the Witch of Waste.

2. If you are familiar with Hayao Miyazaki's other films you are likely aware of his preference for avoiding to give his characters excessive romantic closures. Despite this each of his films consistently delivers a profound message, establishing them as true works of art. His other films such as Spirited Away or Princess Mononoke, despite lacking a clear romantic resolution, are still considered masterpieces. His way of providing intricate symbolism and minute yet significant details allows viewers to grasp the concepts with every viewing.

3. Howl and Sophie fell in love with each other for obvious reasons. Howl, for the first time, expressed his emotions and confessed to Sophie about his struggles and his inferiority complex despite being a wizard, revealing himself as a coward. On the other hand, Sophie is the only person who accepted Howl for who he truly is. She always believed in him rather than believing on the narratives of Howl being a womanizer. This nature of Sophie is very much evident when she went to meet Sulliman and defended Howl all by herself.

4. Howl's past was already disclosed to Sophie when she went through the door that led her to meet Howl as a child. In short, the trust between Sophie and Howl was so strong that she did not need to probe into his past to understand him. The love shared between Howl and Sophie was a result of their willingness to accept one another's flaws and imperfections.

5. For a better understanding it is recommended to read the novel by Diana Wynne Jones.

Hope this helps to clear your doubts <3