I believe Jefferson feared that one country will harm United States because France already sold all of their North American territory, and if they were also to sell New Orleans and Louisiana, the new nation won't have a stand against these possible harms.
Through the Louisiana purchase, Jefferson has taken the opportunity to make America larger, even if it meant going against his republican principles of small government (some would say he exceeded his constitutional power by accepting the deal on his own).
It also gave the U.S. control of the Mississippi River and the port city of New Orleans, both of which were used by farmers to ship their crops and get paid.