I understand this is probably a huge question with no one answer, since it is a period that lasted for almost 300 years and historians don't even agree on the year they began.
But what were the initial reasons for them, then? What was going on in China at the time that caused such an exhaustingly long age of state? What kept these wars going on for so long?
Thank you.
The Warring States Period can be more accurately considered as an extension of the Spring Autumn Period, 250 years that preceded the Warring States period.
Together these 2 periods are considered the "Eastern Zhou Dynasty" period, which began a slow decline of Zhou feudal rule over the non-unified Chinese States until the final unification of China under Qin. This period is known as the "Eastern Zhou", because in the beginning of the period, Zhou ruling clan moved its capitol from Western China to East, which marked the symbolic acknowledgement that the Zhou's power had suffered a serious decline and was unable to defend itself in the West.
This start effectively led to several events of serious geopolitical power shift:
(1) the Zhou Kingdom itself suffered serious loss of wealth and manpower, as its Western capitol was sacked by Barbarian Qiang people.
Zhou Kings were never able to recover from this decline in economic power. And thus, was never able to rebuilt its military to its former might. And other Chinese feudal states became more and more powerful, and Zhou was not able to keep them in check.
(2) As the Zhou Kingdom vacated more than 1/3 of its Western territories in fear of Barbarian Qiangs, the Zhou King struck a bargain with the Baron of Qin, that if Qin could pacify the lost territories of Zhou, that Zhou would cede these territories to Qin for perpetuity.
The Baron of Qin within 3 years would manage to retake the lost Zhou territories. Thus, suddenly, Qin became a powerful feudal State in Western China.
This effectively caused the Zhou kingdom to be now squeezed in the middle by growing feudal states. Over time, the Zhou Kingdom further suffered generations of internal strife, as rival claimants to the Zhou throne were supported by different feudal states, and then the Zhou Kingdom was further carved up by the rival claimants and then subsequently absorbed into neighboring feudal states.
As the feudal states no longer feared Zhou's military power, Zhou could no longer keep the peace between the feudal states.
Small feuds and vendettas led the feudal states into small conflicts during the Spring Autumn period. However, 100's of feudal states of China fought bloody wars, where most of them were destroyed until only 7 remained.
Then, in the Warring States period, these last 7 battled each other. And for a while, they seemed to unable to break the stalemate between them.
The main reason for the stalemate was that none of the 7 kingdom had a clear advantage over the others.
This went on for 300 years, until Qin finally destroyed the other 6 and unified China.