In our daily life, there are many occasions on which we should not hesitate to refuse when asked for help. For one thing, when it is beyond our ability, we should give a direct reply “no”. For another, we the thing we are asked for is unreasonable or even illegal, we should say “no” without hesitation.
However, it is not the case in our real life. Many people prefer to say “yes” when they should say “no”. Some are afraid to lose their face, since they think refusal means their inability. Some people are afraid to offend their acquaintances' face, such as their friends, relatives and so on.
In fact, doing so is quite harmful. If you agree to do the things beyond your ability, the result will only be worse. The other might as well have asked another person who can help. And if you agree to do the unreasonable or even the illegal favors, such as cheating in the exam, you are in fact not helping the other but hurting him. You yourself will get into trouble too.