Season 3: looks
I read the most beautiful comment about the look they share in 3x15 from @hellowherearemypeople “ as soon as Neal asks to speak with Emma alone she looks to Hook. Emma is nowhere near a place where she can let herself think about Hook, or anyone, romantically (she just found out her last boyfriend was a monkey set on murdering her), but she CARES about him - and the concern she has for how he will react in this scene is just beautiful" (you can read the complete post here)
Looking closely at Season 3 they looked at each other on an special way. This season planted the seeds of their relationship; they get to know and trust each other.
Their first kiss is an important moment: in the Echo Caves Killian admitted having feelings for Emma, and Emma was really touched by his declaration.
We have the longing, the desire to take the next step, but Killian wanted to respect Emma’s space and Emma was afraid of Killian’s feelings (no one had looked at her like Killian did) and her own
And the final gif: all the longing, desire and dreams concentrated, but this time they took the next step to their future (and they share the most magical kiss ever)