There are no exact evidence of why dogs develop separation anxiety, but it is more often that the case in dogs have been adopted from shelters, than the ones which grew up fit the same family and haven’t changed environment since he was just a cute French Bulldog puppy. It is important that you know your dogs behavior is not attempt seek attention or punish you for leaving him, but a genuine expression of panic and distress.

As a French Bulldog breeder I can recommend some tips that should help you treat minor separation anxiety: Do not make a big deal when you are about to leave the house or when you get back. Try to ignore him for the first few minutes and then calmly pet him.
When you are leaving, give your dog a part of your clothes that you wore that day, so that he can sense your smell. For a start or in cases of more severe anxiety, you might consider using some calming product. It is not a bad idea to consult your veterinarian and ask for a recommendation.
Choose a safety word you tell your dog every time you are about to leave, so he knows you will be back. During the adjustment period, you might consider the possibility of taking your dog to a pet day care or to your friend or a family member, until you come back.
If your dog has destructive habit of chewing ad scratching furniture when he senses you are about to leave, you should put him in a room where he usually spends the time. If you are living him alone at home, it is better to do it in a room with windows and other distractions, rather than in an empty or dark room. It can’t harm if you give him some toys to play with.

What not to do: Punishment is not an option. It is important that you understand your dog is in a genuine state of stress and his potentially destructive behavior is just a symptom. He is already punished enough without you doing it additionally. Getting another dog also will not solve the problem, because your dog is not lonely. He is just too attached to you. Crating is also not recommendable and it will probably make him even more anxious, which may result in an attempt to escape. Obedience training is also not helpful, because as we said before, your Frenchie is not disobedient and this is not a temper issue. He is just too afraid of you leaving him alone.

If nothing else works, some may advise you to take your dog to work with you, but I suggest you consider that as the last option, after you have exhausted all those we have mentioned above. I hope you these tips will help you teach your dog to stay alone at home, so good luck to you both!