At first, the wars simply made sense and he generally emerged as the victor by standards of the day. An example is the Rampjoor, or Year of Disaster, a Dutch defeat against a combined Franco-English enemy after brilliant diplomatic maneuvers by Louis engineered a total reversal of European politics, securing a French alliance with England.

True financial disaster came withe War of Spanish Succession, which can be regarded as a blunder, but an unexpected one. Louis came to an accord to partition the Spanish Empire with Austria, countersigned by the United Kingdom. He betrayed the accord once the Spanish King surprisingly named a Bourbon as his heir. Today we know he overplayed his hand, but it wasn't obvious at the time. Most of Louis' best marshals like Turenne and Conde had died, and the quality of French horse had severely declined as mounts were lost in past wars. However, France was a victorious country and there was no reason to think she would lose. Surprising defeats forced a long and hard war on the French that cost them dearly.

They cost such a headache because (in brief) under Louis XV, a dilettante King without Louis XIV's personal discipline, or, as the King called it, self-conquest, power decentralized to nobles who formed parlements in local areas and tax collection declined. Louis XV continued expensive wars, such as the Seven Years War, and was powerless as soon as he allowed the parlements to rise and the "Holy" figure of the King to become ridiculed after his numerous affairs to raise taxes.

Source:

Marlborough, his Life and Times.