Why did man join the army on either side during the civil war?

The reason why is because the civil war was fought because of slavery, the north being against slavery and the south liking it, men would fight on they side they believed in. that's why the term "brother against brother and father against son" was created. often households would break apart because either the brothers believed in different things and joined the opposite sides than the other or the father thought different than the son and the fathers would often disown the sons if they both survived the battle.

Well in the south, it was to liberate the people to the unfair tariffs, and keep their slaves. Also to break the tyranny of the Union. The north fought to preserve the Union, and liberate the slaves.