To put it bluntly, Feng Shengxuan had a father-daughter relationship, brother-sister relationship, and soulmate-best friend relationship with Nuannuan. Adding them up together, he became very possessive. Whenever a man appeared by Nuannuan's side, Feng Shengxuan would have the urge to destroy the man. Hence, he assumed that this was love.
Just look at Big Brother Chi Yang. Every time Big Brother Chi Yang saw Nuannuan, especially when the two of them were alone, his lower body would always feel bloated. When he did not get to have her, his eyes would seep with a resentfulness like a huge yearning puppy. After getting satisfied, he would look like a demon who finally had his share of the essence absorbed from the sun and moon. This reaction was normal in a man.