It was the small gestures, like opening the door for her, opening her water bottle so she can drink or bringing sana's favorite foods in her lunch box so they can eat together made her realize that Dahyun was different but she wasn't quite sure what the difference was. 
The realization came to her one day after being asked out by "the hottest guy in the school", the football team captain and kindly declining it before going to her friends who have been waiting for her. When she reached her friends and they started asking about the guy, there was something in Dahyun's eyes that she couldn't really name, the younger tried to hide it but the fake smile never reached her eyes. All Sana wanted to do after seeing that was hug her and never let her go.
Since that day she realized how different from other friends Dahyun's hand feels in hers, she noticed that the way the younger girl looks and smiles at her is softer than what she does with others, she noticed that Dahyun is always there for her, whenever she needs her even when she insists she wants to be alone. And finally she noticed the butterflies or rather the whole zoo in her stomach whenever she's with Dahyun and she doesn't understand how she never noticed that before. 
That one day made her realize she's fallen in love with her best friend but she doesn't know what to do about it...