Ok well if your dog hasn't been clipped then it may get knots in the fur here and it will get sore so trim the hair away in the armpits with round ended scissors. If you use a harness it could be that rubbing the skin, also if he has been clipped at a shop they will shave this area so it could be sore from that as well so rub some antiseptic cream there its ok to use this. Hope this helped