We used CPF to pay HDB. But after we fully paid using CPF after 25 years loan, we sell the house to buy another, why are we still paying the interest in the loan when the CPF money should belong to us? What do you guys think?

You are paying to your future self for retirement.

I think another way of looking at it is that the money within CPF is your own money. Instead of investing in CPF, you are investing in the house. The interest is just the expected returns from investing in the property. So any gains from the sales are back to your CPF.

I think it is just a matter of really poor marketing, calling it CPF loan. In essence, you are using your money to buy your own house, and returning the money you took out from your CPF account, and giving some of the gains back to your CPF as a return on investment.

You're paying yourself the interest that CPF would have paid to you had you not used OA to purchase the flat. So it's still your CPF money at the end of the day. You can use it for your next purchase, right?

Yes it belongs to us, that is why the interest goes back into our CPF account. This way, we will have enough for our retirement.