The main reasons that led to Franco-Prussian war are as follows:
France did not like the growing influence of Prussia
France did not want Prince Leopold to rule Spain whereas Germany wanted him to rule.
William 1 refused to give his assurance when French ambassador asked him to reassure him that he will not support Prince Leopold to become ruler of Spain, Bismark published Ems telegram which provoked French anger.
These were the main factors which led Franco-Prussian war