It was because Ye Jian was Xia Jinyuan's girlfriend. He was thankful that he was strict when training Ye Jian and didn't treat her a certain way just because she was a female soldier.
He had already confirmed their relationship. Even if he wanted to make any advances toward Ye Jian, he had to scrap his plans. A friend's girlfriend was not to be touched. This was his bottom line. As a soldier, how could he cross that line?
Besides, he was not upset to find out that they were lovers. Instead, he felt that their relationship was normal.
Now, he was very sure that he only admired Ye Jian. He didn't have any other thoughts. Even the subtle feeling of happiness that he experienced every time he saw her smile was definitely not because he had feelings for her.
It was not that he had never been in a relationship before, nor had he never been moved. How could he treat the admiration between comrades as the romantic feelings between men and women?