Unexplained weight loss==is it always cancer or something bad?

Depends on health. This is a nonspecific finding that can mean several things. It could be cancer, or diabetes, of infection, or something else. Hyperthyroidism can do it. It also can be due to metabolism pills for weight loss. Other symptoms, and findings can point to a cause. It would be wise to see your doctor to find an explanation.
It is not good. It is not good to have unexplained weight loss. It indicates serious medical or psychiatric illness. It is not necessary to be caused by cancers. Conditions that can explain- hyperthyroidism, diabetes mellitus, other endocrine problems, i.e.Adrenal insuficiency, malignancy, chrohn's disease, mesentary artery syndrome, hiv, advanced cardiac, pulmonary, kidney disease, depression, drugs.See your md.
Talk with your doc. Yes, there are many causes of weight loss including hormones like too much thyroid. Badly controlled diabetes is another. Some reasons - like cancer - are more serious. Please talk with your doc about why you or someone you know is losing weight. Great question!