Jealousy isn't something Louis feels...He idolizes his brother and is loyal to him until the very end, so he's not used to this feeling of envy and jealousy growing within him...He had never once felt this way before you showed up...

His brother is rather fond of you and while William has no intention of getting involved with you, Louis can't help but feel that his brother is rubbing the extra time he spends with you in his face...That's why he's asking his brother calmly to leave you be...