From The Federalist:
The lack of nationwide reciprocity also creates a bizarre legal paradox. For almost a few hundred years now, American courts have recognized that interstate travel is a right possessed by all Americans and guaranteed by the Constitution.
Yet when a state does not offer recognition for a citizen’s out-of-state concealed-carry permit, the right to travel comes into direct conflict with the right to self-defense. In other words, under current law an American citizen can lose the constitutional right to self-defense simply while exercising his or her constitutional right to interstate travel.