Like the FL is supposed to portray a girl in her 15-16... and the ML is close to his 40-45-ish...
Something along with the ML basically just sees the FL as an innocent, naive child/teen and its not until she grows both in age/character/experience and whatever he starts getting romantic feelings for her.
Before he perhaps saw himself more of a parent/older brother/uncle -figure for her. So he didn't get jealous if she had a lover and wanted to go after him, instead perhaps he encouraged her to experience her own youth and find her own happiness.
