I think the logic goes something like this: If we don't expect greatness, if we don't expect to find love and be healthy and successful, well we're not going to be disappointed when these things don't happen.
他们的逻辑大概是这样的： 如果我们不期待成就什么伟业， 不期待找到爱人，身体健康，取得成功， 我们得不到这些东西的时候也不会特别失望。