Why did Holling think Romeo and Juliet were stupid?

Much to his surprise, Holling starts to become a big Shakespeare fan, and soon starts quoting out loud from his favorite plays. He also uncovers a good deal of practical wisdom and more than a few important life lessons from the Bard, structuring key parts of his life around themes found in Shakespeare.
His relationship with Meryl Lee is particularly colored and shaped by his reading of the plays. Initially, things are pretty fraught between Holling and Meryl Lee, so much so that he calls her a "blind mole," an insult he's picked up from reading The Tempest. He also acted like a jerk about his father winning the Baker Sporting Emporium contract, which Meryl Lee holds against him. So a few months later, when Holling asks Meryl Lee out on a Valentine's date, she initially turns him down flat.
Getting the brush-off from Meryl Lee pretty much confirms how Holling has been feeling about Romeo and Juliet, which he's been reading with Mrs. Baker. He thinks that Romeo and Juliet were stupid because they wouldn't have acted the way that they did. They certainly shouldn't have killed themselves; they should've just defied their parents and run away to Mantua.
But Holling soon manages to sweet-talk Meryl Lee into going out with him, using (naturally) Romeo's words to Juliet from Act II Scene II:

"[T]here lies more peril in thine eye
Than twenty of their swords."

Holling takes Meryl Lee to a play for their big Valentine's date. No prizes for guessing which one they go to see. It's a great experience for both of them, and it changes how Holling feels about Romeo and Juliet. Thanks to Meryl Lee he now understands that Shakespeare was trying to express something about what it means to be human. He also comes to realize that Romeo and Juliet were star-crossed lovers, fated to fall in love and equally fated to die as they did. Now he understands why they didn't run away to Mantua, and why they died in such tragic circumstances.