When do you think Leo Fitz realized he was in love with Jemma Simmons? Was it a particular moment? A particular episode?
I feel like no words in the English language can adequately describe the progression of his feelings (notice I said progression, not moment. I don’t think his realization comes as a torrential downpour. Rather, it was an incessant drizzle that eventually soaks his everything), so I will just let these annotated screencaps work their magic.
First of all, I think the fandom has come to a consensus that for the first few episodes, Fitz harbored a crush on Skye. Even at the beginning of FZZT, a major turning point for FitzSimmons, we can still see him trying to make Skye laugh, agreeing with everything she said, and actively seeking common points with her.
Everything changed when the fire nation attacked the very moment she contacted the Chitauri virus.