While it could be argued that it began in the early 1700s, a more solid date would be in the early 1800s. The tensions between the two regions was centered on slavery: the economics, the morality etc. Beliefs on slavery in America can be divided into two phases, pre and post 1800. Before 1800, slavery was termed the "peculiar institution." Few people outright defended it and instead said that it was this sorta weird, but convenient thing that will go away sooner or later.

By 1800, however, one crop in particular began to become extremely valuable: cotton. Slavery transformed from this weird thing that nobody wanted to talk about, to a way of life for a society. u/Georgy_K_Zhukov, had a phenomenal response to a question the other day on beliefs on slavery of the average Confederate soldier. For the north, slavery was a nice source of cheap cotton for factories. For the south, slavery was a way of life.

These differing beliefs on slavery were the root of tension between the North and the South, and they really only began to get heated after 1800.

/u/colorfulpony provided a good answer, but I think there is more to the story. The reason the tension began to build so much can be widely attributed to the Second Great Awakening. From about 1800 to 1850, religious movements were booming across the United States. Coming off of the Revolution and embracing democracy, Americans were eager to establish their religious identity. This rampant expansion and development of religion brought with it a couple new ideas- Postmillennialism and Free Will/Arminian Theology. These two major theological shifts eventually caused many Christians to "pursue the millennium." This effort helped grow the previously planted grassroots of abolition exponentially, and really turned the dial of the tension between the North and South up to 11. Some key figures to look into would be William Lloyd Garrison, Richard Allen, Jarena Lee, Charles Grandison Finney, Lyman Beecher... the list goes on. The Second Great Awakening also saw the grassroots of suffrage, prohibition, and widespread educational opportunity.