This is the story of a young girl's difficult, painful enlightenment to some basic truth's about life on earth. For instance, the reader could easily interpret this entire plot as an existential crisis. In that case, Drosselmeyer's gift of a clockwork world would be a way of showing Marie that chaos is necessary in life, or else everything is repetitive and boring (like that toy quickly becomes). She much prefers the dark chaos of her imaginary world where the stakes are life and death, where people take tragedy with communal support, and where (most importantly) no one is alone. Marie is probably mourning the death of her parents, which is implied in that they live with Drosselmeyer who is their godfather.
This newfound awareness of tragedy and death makes Marie's imaginary world into a tool that she uses to ask difficult questions about life than she can otherwise handle emotionally. Look for a moment at just how violent this seven year old's imagination is—she imagines a world of bitter warfare between two antithetical forces. She imagines the pain of loss for the community each time someone dies. Interestingly, she also imagines funeral rites and spells of mourning when bad things happen, and when the toy community loses someone, they all rally together to support one another.
Based on this, the reader can guess safely that Marie is struggling with something specific. She wants Drosselmeyer to be in community with her, so that he can help her more directly with the pain of her life. She needs help and support from someone, and these stories are like cries for help through which she invites Drosselmeyer into her mind and emotions. Sadly, he misunderstands this, and he has a hard time relating to her, so she stays fairly lonely in her godfather's estate.
