What could be the reason for constant weight loss?

Lowered calories, diabetes, infection, diarrhea, malabsorption, depression, thyroid disease. See your doctor, this list goes on and on

Generally we approach this symptom by asking whether you have a normal appetite or not. Weight loss despite good appetite and food intake occurs in diabetes & overactive thyroid, presence of tapeworm. Poor appetite is seen in malignancy, chronic infection, psychiatric disorders, gastrointestinal diseases,medication side effects for example. See a physician for examination & testing.