This is the 4025th most frequent Korean word.
이유에
The Korean word "이유에" means "to the reason" or "for the reason."
'이유에' is used here to indicate the 'reason' as the subject in discussion that has a certain attribute (a contradiction).
그의 말을 믿지 않는 이유에는 심각한 모순이 있습니다.
There is a serious contradiction in the reason not to believe his words.
'이유에' is referring to 'the reason' as a reference to elaborate on causes behind it.
우리가 늦은 이유에는 몇 가지 이유가 있습니다.
The reason we are late has a few reasons among it.
'이유에' demonstrates the factors ('열정와 끈기') that are included in the 'reason for success'.