godan verb (-ru), intransitive verb
miss someone; feel lonely; long for
Expresses visible or audible signs of loneliness, missing someone, or wanting company. Often used when someone's behavior shows they are feeling lonely.
一人になると、すぐ寂しがる。
When left alone, he/she quickly shows signs of loneliness.
彼女は友達が帰ると寂しがって泣いた。
She missed her friend and cried when they left.