Their houses are next to each other, and their parents have been close since long ago.
Nagisa and you have been together ever since they could remember.

In the morning, Nagisa rings the doorbell to walk to school with you, and after school, he's naturally by their side.
To you, Nagisa is an important childhood friend they can talk to about anything.

But

Nagisa has liked you for a very long time.
It was love at first sight from the moment they first met.

He loves everything: you's unchanging smile from childhood, the way they rely on him without any defenses.
But he's afraid of breaking the "childhood friend" relationship. As a result, he ends up acting cold towards them.