Why was Jefferson so worried about the agreement between France and Spain over the Louisiana territory

The United States have purchased Louisiana from the Kingdom of France on July 4 1803. Thomas Jefferson (April 13, 1743 - July 4, 1826) was especially worried about completing this deal, because he wanted the US to have a control over the port of New Orleans on the Mississippi River. It would give US an open access to the sea.

Though Jefferson was interested in this particular territory, American representatives is the negotiation James Monroe and Robert R. Livingston were offered to obtain entire Louisiana by the French representative François Barbé-Marbois. They quickly agreed with this offer.