The proton is a fundamental constituent of the atomic nucleus, one of the Universe’s most common particles and the fuel of stellar furnaces. Nevertheless, many features of the proton, even its fundamental entities such as its mass, charge radius and spin are not yet well understood. Most of these issues if not all, however, appear to be interrelated to each other. In this talk, I discuss another piece of the proton puzzle, “isospin asymmetry of the proton sea”, that may offer some insights to unravel the proton mysteries. The isospin asymmetry, or more broadly known as the flavor asymmetry, of the proton sea has been observed in worldwide experimental facilities, typically at CERN, Fermilab and DESY. The new experiment is also forthcoming at Jefferson Laboratory with the 12 GeV upgrade of the electron beam energy. I’ll discuss why this asymmetry is puzzling and how it may offer in-depth understanding of the internal structure of the proton from the perspectives of the chiral dynamics in QCD.