Hey mate, don't listen to them. You have no idea what a great strength you have. People are advised to have a smiling face. I can't count for you what are the advantages of having a smile on face always, you better Google that. In my personal experience I smile rarely in office so everyone calls me Serious Guy. Now you tell me SHALL I GO INTO DEPRESSION??? When people see something good you have and they don't have they start pulling your leg. Your colleagues think they can't have have smiling face like you. My colleagues think they can't be serious at work like me. Tell them with your smile GO TO HELL 🙂. Live your life and smile always. All the best.