As many have said, co-signing is always a bad idea. I will occasionally loan money to family or friends but only after evaluating their credit-worthiness. As mentioned above, what do you know that the bank doesn't know? You should evaluate the loan on its merits and try to not let emotion come into the decision, and you should never loan more money that you can comfortably lose.
For example, my brother and SIL were trying to have a baby and banks generally won't loan money for IVF (because it's hard to repossess a child and it doesn't always work). They ended up getting a "fertility loan" at an awful rate. When I heard about this I offered to loan them the money to pay off the high-rate loan so they wouldn't get so screwed on the interest. Several factors went into my decision to make the loan:
- Overall they have decent credit with no loan defaults
- My brother and I own a house together, so I already rely on him to make timely payments on something that will screw my credit if he doesn't and he has never let me down.
- His monetary temperament is similar to mine. We both were raised to be very frugal and don't spend money on needless things.
Even though I was pretty sure he would make payments on time, I wrote up a loan agreement that specified the payments and payback schedule. It helps both the lender and borrower when the terms are clearly defined. If you make a loan that is "pay me back when you can", don't expect to ever see that money again. So far my trust had been justified, he has been making regular payments for 18 months without issue and my new nephew is a source of joy in my life so I'm happy I was able to help them out.
It's nice to be in a position to help out family and friends financially but I got into this position by being responsible with my money and I refuse to help people who are in the position they are in because of their irresponsibility. Evaluate any requests like this on its own merits and again, never co-sign.