古代社会经常出现叛军的原因有很多。首先，社会不公平和民众的不满是导致叛乱的主要原因之一。在古代社会，社会阶级划分明显，贫富差距极大，统治者常常压迫和剥削底层人民。无法忍受生活的艰辛和不公，一些民众选择了反抗，组成叛军。这种反叛行为能够让他们争取更好的生存条件和社会地位。

The ancient times witnessed the existence of rebel armies due to various reasons. Firstly, social inequality and people's discontent were one of the main causes of rebellion. In ancient societies, there was a clear division between social classes, and the gap between the rich and the poor was enormous. Rulers often oppressed and exploited the lower class. Unable to bear the hardships and injustice, some people chose to resist and form rebel armies. This rebellious behavior allowed them to strive for better living conditions and social status.

此外，政治腐败也是叛军兴起的重要原因之一。在古代，很多统治者腐败堪比世界上任何一个时期。他们以权力为工具，为自己谋取私利，而忽略了人民的需要。这种政治腐败导致社会动荡，人们失去了对政府的信任，进而加入叛军，试图推翻不公正的统治。

Furthermore, political corruption was also a significant factor in the rise of rebel armies. In ancient times, many rulers were as corrupt as in any other period in history. They used their power as a tool to seek personal gains, disregarding the needs of the people. Such political corruption led to social turmoil, eroding people's trust in the government, and consequently driving them to join rebel armies in an attempt to overthrow unjust rule.

叛军的形成还与战争和边界争端有关。在古代，战争和边界问题屡见不鲜。国家之间的领土争夺常常导致内乱和分裂。被压迫的人民或被侵略的民族为了保卫自己的利益和尊严，组织起来抵抗敌人和政府。这些抵抗行动就演变成了叛乱和叛军。

The formation of rebel armies was also related to wars and border disputes. In ancient times, wars and territorial conflicts were frequent. Territorial disputes between nations often led to internal turmoil and division. The oppressed people or invaded ethnic groups organized themselves to resist enemies and governments in order to defend their interests and dignity. These resistance movements eventually evolved into rebellions and rebel armies.

最后，宗教和意识形态的冲突也是叛军产生的因素之一。古代社会虽然信仰多样，但宗教和意识形态冲突也时有发生。当统治者试图强行推行特定的宗教信仰或意识形态时，往往引发一系列反抗行为，其中包括了叛乱和叛军的形成。人们为了捍卫自己的信仰自由和价值观，选择了投身到叛乱的行列中。

Lastly, conflicts between religions and ideologies were also factors contributing to the creation of rebel armies. In ancient societies, although there were diverse beliefs, religious and ideological conflicts also occurred from time to time. When rulers attempted to impose specific religious beliefs or ideologies by force, it often triggered a series of resistance actions, including the formation of rebellions and rebel armies. People chose to join rebellious movements in order to defend their freedom of belief and values.

总而言之，古代社会之所以会存在叛军，是因为社会不公、政治腐败、战争边界问题和宗教意识形态冲突等多种因素的综合作用。人们对不公正和压迫感到愤怒和不满，因此选择了反抗。这些叛乱行动在一定程度上改变了古代社会的格局，并对历史产生了深远的影响。

In conclusion, the existence of rebel armies in ancient societies was a result of various factors, including social inequality, political corruption, wars and border disputes, and conflicts between religions and ideologies. People felt angry and resentful towards injustice and oppression, thus choosing to resist. These rebellious actions to some extent changed the landscape of ancient societies and had a profound impact on history.