Why will Aarav not let Dadi tell him her PIN?

Dadi: Aarav, come here. The vegetable man says I can pay him with my phone. Show me how.

Aarav: Okay, Dadi. First open this app. The one with the blue circle.

Dadi: This one?

Aarav: No, that's the calculator. This one. Good. Now press "Scan".

Dadi: It's showing my face.

Aarav: That's the wrong camera. Turn the phone round. Now point it at the black and white square on the card he gave you.

Dadi: It made a sound! Did I pay?

Aarav: Not yet. Now type the amount. How much is it?

Dadi: Sixty-five rupees. Six... five. Now?

Aarav: Now your PIN. The four numbers. Don't tell me what they are.

Dadi: Why not? You're my grandson.

Aarav: Because a PIN is secret, Dadi. You never tell anyone. Not even me.

Dadi: (typing slowly) There. It says "Paid". A green tick!

Aarav: Well done. Now show him the screen.

Dadi: Wait. Can I see where the sixty-five rupees went?

Aarav: Yes, press "History".

Dadi: (reading carefully) Sixty-five rupees, to Ramesh Vegetables. Good. Now teach me how to check my balance, because next week I am going to buy a sari.