This is a matter of shells, each period in the periodic table represents number of shells of its atoms, the 1st period represents only one shell which is already the valence shell, and in this period we only have Hydrogen and Helium. According to the electron configuration, the first shell only needs 2 electrons to be complete, this is not true for other shells like 2 or 3, these two will follow the octet rules most of the time, needing 8 electrons to be stable
electrons configuration:
1s2