What is the root cause of the animosity between the Night's Watch and The Free Folk?

The wildlings are a real threat to people of Seven Kingdoms because of their raids:

Raiding south of the Wall is a large part of free folk culture. Raiders start at a young age, as little as twelve years. Raiders either climb the Wall or use little boats to cross the Bay of Seals around it.

Over the decades, with the weakening of the Night's Watch, the free folk have found it much easier to either climb the Wall or paddle small boats through the Bay of Seals, growing bolder they raid as far as the Umber lands, the northern mountain clans or even Bear Island, instead of the usual villages and holdfasts in the Gift.

Since the White Walkers where not seen for eight thousand years, people of Seven Kingdoms forgot that they were the original reason to built the Wall. Instead, they came to believe that it was built to protect them from the wildlings. The absence of White Walkers also caused the Night's Watch to change its focus as the wildings' raids became the only actual threat.

Apart from the raids issue, centuries of forced separation lead to many cultural differences, but most importantly prejudices on both sides. The wildlings are seen by the civilised people (that the Night's Watch recruits from) as lawless savages and barbarians. On the other hand the Free Folk sees the southerners as weak kneelers, who are subject to people who they didn't choose to rule over them (i.e. lords). They are also well aware of the fact that their inferior position of being behind the wall is only a matter of misfortune, as they are not worse from the people of Seven Kingdoms in any way.

Based on the above it's quite understandable that the Night's Watch and Free Folk fight each other and many centuries of this fight lead to a great amount of hatred on both sides.

Once the Wall was raised by the people of Westeros to protect the lands from the White Walkers, the Undead plague and all.

This Wall separated the people into those 'north of the Wall' and the 'normal' people which means the regular inhabitants of the North Territory. This means, the people once of same culture and identity began to develop differently.

Later, the Wildlings/Free Folk considered it their right to ambush and plague the people south of the Wall - just because they were their only neighbours which could be raided and also, because 'the Southerners' (as the Free Folks call ALL people regardless of their origin be it the North or even Dorne) once abandoned them and left them behind exposed to the threats of the long winters, the Undead and the White Walkers.

I see this as the population of America before Europeans arrived. There were wildings up north and also a war with the walkers. I think it is safe to assume that the war affected the wildlings, they wanted to head south where it was warm, the population of the south (Westeros) didn't want them inside their community since they were primitive and violent, they constructed the wall and they locked themselves from the walkers and the wildlings.

I'm pretty sure that what we are seeing at the moment is the same thing that happened when the whites walkers last came. They marched south, wildlings scared from them, moving ahead of them to escape the fatality, roaming down south, hungry and thirsty, stealing everyone and be declared as wildlings (barbarians, killers etc...). Then the men of Westeros made the wall and "won" the conflict, killing most likely a lot of wildlings. So the hate is equal on both sides, but the real bad guys are our beloved 7 kingdoms characters, they did lock the wildlings away from the "civilization". And by locking them away, they were doom as soon as the walkers rose back, which is what is happening at the moment. At least they had Jon Snow to prevent a carnage.

Edit 1: To awnser the "root" of the animosity, I would say the root is the arrival of the first White Walkers, that drove the wildlings to do things for survival that the 7 kingdoms condamned and concluded they couldn't join their realm, they had to be locked away.

Edit 2: This is from a perspective that never read the books, it might be explained but I'm fairly sure this was not addressed in the books. Correct me if I'm wrong :)