Why did Jefferson find the terms of the Louisiana Purchase treaty a bit embarrassing?

Because he contradicted the Constitution. The Constitution says that it's up to Congress to add a new part of the nation, NOT the President.

Jefferson was interested in buying the Louisiana Purchase because he wanted to make the United States bigger, well-fit, and stronger. He also was interested in the buying the land because he was fearful that foreign countries would control the New Orleands port. Jefferson wanted to secure the Port of New Orleans which to this day still remains as one of the most important ports in the Country.