When should a dog be euthanised?

I think it's time when either the pets in extreme pain qauility of life is deminished or an animal is a danger to other pets or people. I think a pet owner sometimes needs to relize putting off haveing your pet euthanised is not for the pets sake but for there own, you love the animal and feel like your commiting murder, but this is a decision as a pet owner you must be prepared to make because you love you pet and don't want it to suffer or hurt someone. I can understand how hard it is because I had to make this decision with my golden retreiver Mandy which I had for 13 years to make it worse it was right after my daughter lost her fight for life with lung cancer.