Answer: For starters, you teach them to deal with tears and disappointment. By tamping down their hopes, you're trying to prevent tears and disappointment, which is a different effort entirely.
If your kids' hopes are unrealistic, then they're going to find that out when the things they hoped for don't happen; life plays the realist role brilliantly, without your help.
Broken hearts hurt, yes, and a brokenhearted kid means a parent with extra work to do. But a pre-emptive visit from Debbie Downer doesn't make things hurt any less, or make the mop-up easier. Getting rejected is going to hurt no matter what, but when Mom feels compelled to notify you that you're probably going to get rejected, that's an additional kick to the confidence.
Instead of telling kids what they probably won't do, teach them what they can do: practice hard, study hard, rehearse daily, treat loved ones well, pursue happiness.
Even better, notice and praise their hard work and its (guaranteed) rewards.
And, be the safe place they can land when the coaches/teachers/employers/directors/sweethearts of the world stick a pin in their hopes.
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