Why? Loans are not (only) given by the government. I should be free to loan money based on whatever collateral I want. If I want to loan money with my friend

Why? Loans are not (only) given by the government. I should be free to loan money based on whatever collateral I want. If I want to loan money with my friend's prized guitar as collateral, how does that mean that the government should tax my friend's guitar?