Causes of weight loss in your dog
The erratic weight loss could because of various reasons. Obviously, our dogs cannot tell us about their issues directly so we have to decipher it ourselves. There are certain causes which are controllable if identified at earlier stages & other are unfortunately beyond our controls.
Some controllable causes are:
1: Diet: Watch out what your dog is eating. Is it providing the proper set of nutrition which it requires at his or her age? Do the food we are serving to our beloved dog come from a genuine brand? Does it actually contain all what our dog deserves and needs to thrive?
2: Stress: The most important question is what can actually cause stress to our dog? It could mostly because of fear or loneliness. Sometimes, the stress is a result of our treatment, or it is because of other external reasons. But it is very likely that the dog may develop anxiety issues because of its breed or for no plausible reasons.
3: Living condition: The living condition can harm its health more than anything. How often it our dog is getting a bath? How often it is going out? Where are you allowing the dog to sleep & with whom it is spending time with?
4: Environmental conditions: The weather, temperature, humidity, or any external factor that may affect the health of our dog. Just like humans’ dog do come under the weather or when they expose themselves too much to scorching sun then it will directly halt their growth.

5: Tapeworm or Parasite: It is very likely that our dog is losing weight because of internal worms that are eating up it from inside and stunting its growth like anything. In this case, the dog will give visible gestures that could help us understand that whether there is anything which is bugging it from inside. Looking at the Dogs Poop may help you determine if the dog has worms. Its a very good idea to deworm your dog at a vet recommended schedule.
Some not very easily controllable causes are:
1: Chronic Diseases: If a dog has any issue with its lungs, kidneys, heart and with any vital organs then it can cause sharp decrease in its weight. Another possible could be diabetes & cancer as these diseases would result in reducing your dog to a mere skeleton.
2: Age: When we are adopting any puppy or an adult dog then we must be assured about its life expectancy. With age comes fragility and this varies from one breed of dog to another. The aging factor can cause irrecoverable muscle issues and bone fragility which has no cure, but death itself.
3: Dental Problems: This is one major cause and at times it is not possible treat it. Modern vets now have full facilities to address this issue, but if your dog has any defect from its birth then it might not be possible to treat it.
4: Pregnancy in female dogs: Pregnancy in female dogs is one of the several reasons which could result in sharp decrease in weight.
5: Breed: There are certain breeds which have this innate issue that they cannot gain adequate problem or they are not just fit in the environmental conditions of your area.