You should smile back when someone smiles at you.
Rob is working in his office when the new girl, Cleo walks in grinning and says hello.
Rob wants to acknowledge Cleo.
Rob stops working and stands up to shake Cleo's hand while giving her a friendly smile.
Rob looks up from his computer and makes eye contact with Cleo, but just gives a quick wave.
Cleo feels welcomed and think she will enjoy working in this office.
Cleo sees Rob as dismissive and unapproachable which makes her uncomfortable on her first day.