As a co-signer or guarantor, you're not just vouching for the borrower's character — you're legally on the hook to repay the loan if they default, exactly as if it were your own loan. This isn't a formality; lenders can and do pursue guarantors directly for repayment when the primary borrower fails to pay.
It also affects your own credit profile and borrowing capacity, even if you never make a single payment. The loan typically shows up on your credit report as a liability, which can reduce your own eligibility for a loan you might need later, since lenders factor in your total exposure, not just loans in your own name.
Before co-signing, it's worth having an honest conversation about the borrower's repayment plan and what happens if their circumstances change — and treating the decision with the same seriousness as taking out a loan yourself, because legally and financially, that's essentially what you're doing.