The whole belgian campaign was a surrealistic waste.
Things were clear to anybody. Napoleon acted just like a selfish and careless gambler. In which he was, however, helped by the stupidity of the restored Bourbons.
Actually, Louis XVIII wasn't all that stupid; but he made one crucial miscalculation.
Believing (correctly) hat conscription was unpopular, he abolished it in a bid to gain favour. However, the Law of Unintended Consequences chipped in. Releasing the conscripts left an army consisting mostly of those who had little to go back to in civilian life, and saw the army as a career - precisely the ones with most to gain by Napoleon's return.
Paradoxically, Louis would have been better off with unwilling soldiers than with willing ones - the kind who viewed military service as a jail sentence, and were impatiently chalking off the days till their release, like a convict on his cell wall. Had Napoleon met an army of such men at Grenoble, they would have been horrified at the prospect of being dragged off to war again, and would probably have shot him down without the slightest compunction.