Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Someone asks you to co-sign a loan. How to reject & say “no” nicely or politely?
Simple and straight-forward.  "I'm sorry but I don't co-sign loans.  I've heard horror stories (or had bad experiences if you actually have) about these things going bad and ruining friendships.  Your friendship is more important to me than you getting this car/stereo/whatever."   You could go on to explain that it's not necessarily a lack of trust in them, but the problem could be cause by things beyond either of your control.  Let's say there's an error at the bank and his payment doesn't get processed on time and it hits your credit score.  Next thing that happens is your credit card company sees the change in your score and jacks up the rate on your card.   Neither of you did anything wrong, but now instead of him just fighting with the bank about the payment not getting processed on time, you are having to fight with your credit card company.  You are both in an awkward situation.  You might get pissed at him (you could make this out to be a failing on your part) even though it wasn't his fault.  Or he might be embarassed to come around even though you know it wasn't his fault and aren't pissed at him.