After all the things that Sam had seen Gandalf do, mentioning only the fireworks shows that the he loved the fireworks so much.
Sam's recital using the fireworks shows that this was the most memorable thing that he liked about Gandalf.
According to him. he described the fireworks as the finest rockets that he had ever set eyes on. He also described them as stars of blue and green that fell like it was raining flowers.
In summary this shows that Sam loved to see the fireworks.
