What is the reason for the asymmetry between the twins in the twin paradox?

Abstract

The true difficulty of the twin paradox does not reside in the algebra that shows that the traveling twin ages less than the twin who stays at home. The truly startling part of the paradox resides in the much more difficult question why the argument cannot be reversed by symmetry, because there is no such thing as a preferred reference frame, and motion ought to be relative. Can the traveling twin not claim with equal rights to have stayed at home while the other twin has made the journey? In trying to answer this question something very simple has been overlooked. In drafting the protocol which defines the journey, we unwittingly pick a preferred reference frame, because we define the protocol with respect to a given frame, which thereby becomes special. It is this selection of a special reference frame which breaks the symmetry between the twins.