Why does he have such a hard time deciding which instruments is his favorite

Everything sounded so good, and the music was so cohesive.... he simply couldn't choose.

From the text:

All of the instruments blended up together and, just like that smell in the library, you couldn't tell which one was your favorite. First you'd say it was Mr. Jimmy on the trumpet, then Do-Doo Bug's trombone would make you think it was the best, then Dirty Deed would make the piano sound like water hitting big rocks and you'd know there wasn't anything that sounded that good until Steady Eddie would make the saxophone sing and talk and dance around everyone else and you'd swear that was the only sound you'd ever want to hear again. All the while Herman E. Calloway and the Thug kept everything moving by making the drums and the giant fiddle pound out a soft steady beat, like someone's heart turned way up loud.