Why do you think the expression is the "apple of your eye" and not the zucchini of your eye. It's because apples are good and zucchinis aren't, that's why!.
An apple a day keeps the doctor away and we like going to the doctor almost as little as we like eating vegetables. But doctors want your business, that's why they often recommend eating vegetables...
If someone tells you that you're a peach, that's a good thing right. But who wants to be told that they're a squash. Nobody.
Remember the line from the musical Oklahoma in the song "I can't say No:"
S'posin' 'at he says 'at yer lips're "like cherries"
Having lips like cherries is flattering, right? Would you like lips like eggplant? Of course, not. Cheeries = good. Eggplant = bad.