The concept of Manifest Destiny refers to our desire to expand from the Atlantic Ocean to the Pacific Ocean. We had a right to expand across our continent from the Atlantic Ocean to the Pacific Ocean.
It is normal for a country to want to grow. As our population grew, we needed more land on which to live. We also needed to make sure the needs of our people were met. For example, the farmers in the West around 1800 needed to get their goods to the marketplace. When Spain and France restricted our use of the Mississippi River, we needed to try to get the land that would allow us to use the Mississippi River and the port of New Orleans. It was natural for us to agree to make the Louisiana Purchase.
Other opportunities allowed us to grow. Texas wanted to join our country as a state. Many Americans lived in Texas, so this would be a natural fit for us. We also had claimed the Oregon Territory along with Great Britain. It made sense to agree to divide this land with Great Britain so we could have part of that territory. While our methods of getting the southwestern part of the United States might be questioned, it was normal for a growing country to seek new opportunities for expansion and for growth. The war with Mexico gave us an opportunity to get this land.
Thus, we did have a right to expand from the Atlantic Ocean to the Pacific Ocean. It was something that was natural for us to want to do.