Why did everyone in the north believe that southern secession was going to be a temporary, fleeting development?

Both the North and the South thought the war would only last a few weeks to months. The North thought that as soon as the South saw the devastation of the battle field they would give up because they wouldn't want to lose so many men. The giving up slavery would be worth saving thousands of lives. The SOuth thought they could wait out the North. Between having the North tire itself out and key battlefield victories, they thought that the popularity for the war would wane and the North would simply give up. Neither side was correct and both were set in the idea that they would be free or bring the nation together. No matter the cost in human life.