✔ 最佳答案
According to Oxford Advanced Learner's Dictionary,
'Should' is used after that after many adjectives that describe feelings.
Examples:
I'm anxious that we should allow plenty of time.
I find it astonishing that he should be so rude to you.
(Ref:
http://oald8.oxfordlearnersdictionaries.com/dictionary/should)
The expression of 竟然 does not lie in the word should be in the meaning the sentence conveys. It is common to use 'should' after adjectives that describe a beyond-expectation kind of feelings, such as disappointed, surprised, amazed, furious, etc., just to mention a few.
2013-12-07 17:48:55 補充:
erratum:
The expression of 竟然 does not lie in the word should but in the meaning the sentence conveys.