Low Carb Diet
Harry Potter and the Cursed Child would like you to think that Albus Potter is moody because he's a) fifteen and b) he thinks his father doesn't love him. But there's a third factor that we think might be the most important factor affecting his mood: he doesn't get to eat candy.
Hey: we're grumpy if we have to make it to 4 p.m. without at least one Swedish Fish. And we don't live in a world full of Chocolate Frogs and Toothflossing Stringmints.
Albus' sugar deficiency might also be the main reason he becomes friends with Scorpius: the boy gives him candy. Scorpius even has a song,
SCORPIUS: Sweets, they always help you make friends. (1.3.22)
The lyrics need a teensy bit of work, but it's true…even if it does conjure up images of creepy old men in unmarked, windowless vans.
After Albus returns from the past, and right before Harry tells him never to see Scorpius again, Harry gives him chocolate. He thinks he can soften the blow of telling him "you will be alone forever" with a chocolate bar. Is this all part of Harry's plan to keep his son from never having sweets? To condition him into thinking eating a chocolate bar leads to terrible news?
We discussed Harry's defects as a parent in his character profile (go check it out) but we saved this one for its own special section. Not letting his son have sweets at all—especially in a world that includes such deliciousness as Ice Mice and Cauldron Cakes—is almost as bad as feeding his son a diet of 100% Butterbeer.