Question

Why the electron affinity of alkaline earth metal all positive? ???

Answer 1

because they are metals and metals tend to lose electrons to form cations that's why the electron affinity of alkaline earth metals is positive.
:)

Answer 2

The general outer shell electronic configuration of alkaline earth metal is ns square which is stable the extra electrons should enter the n p subshell for this energy is required so the electron affinity of alkaline earth metals is positive