There were a number of causes of the One Hundred Years War. Foremost was the question of inheritance and titles among kings. Due to complicated family lineage and ties between the French and English royal families, King Edward III of England was also a vassal of King Philip VI of France. The origin of this inconvenient situation began in 1066, when William, Duke of Normandy, seized the English throne. His descendants continued to rule England while still technically being vassals of the French kings.
Threatened by having another king hold a fief in his realm, Philip moved to occupy Edward's land in Guyenne in 1337. Edward responded by claiming succession to the French throne and invading Flanders, where the English had trade interests. Fighting continued on and off for the next century as English kings continued to claim sovereignty in France.
To complicate matters even further, France had a long-standing alliance with Scotland. The French had agreed to support the Scots if England tried to invade that country. The English had long-held designs for conquering Scotland, but they could not do that if they had to contend with the French at the same time. King Edward decided that if he was to invade Scotland he must first neutralize the French as a threat.
The French and the English had also been feuding for some time over control of the English Channel. Both sides had funded marauding privateers to plunder the others' ships and disrupt trade. The French and the English hoped that this conflict would settle the matter over dominance of the Channel.