理由英语是reason。
例句：
1、这个结果使我们更有理由保持乐观。
This result gives us all the more reason for optimism.
2、你没有理由忧虑未来。
You have no reason to be apprehensive of the future.
3、他以精神失常为理由获判无罪。
He was found not guilty, by reason of insanity.
4、你完全没有理由不去。
There's no earthly reason why you shouldn't go.
5、我为什么要帮你？给我一个充分的理由。
Give me one good reason why，I should help you.
长亭不再送别
