Ce3+ is easily oxidised to Ce4+. WHY?

 Cerium has electronic configuration: Xe 4f¹ 5d¹ 6s²

Ce³⁺ = Xe 4f¹ 

which is easily converted to the Ce⁴⁺. As after loosing one more electron it will convert to the stable empty electronic configuration.

Ce³⁺ = Xe 4f⁰