What are the reasons that Socrates gives in Plato's Republic for philosophers not wishing to rule?

The background to Plato's treatment of rulership in the
Republic is both philosophical and biographical. On a biographical level, the account of
Plato's sojourn in Syracuse in Epistle VII suggests that Plato's personal experience was
that rulers were not able to devote themselves to philosophy and that the quest for
philosophical truth was incompatible with the compromises necessary for
rulership.

For Plato there is considerable tension between
the need for philosophically trained rulers, who are unselfishly devoted to knowing and
practicing goodness, justice, etc. and the fact that true philosophy finds its
fullfillment in the theoretic life of contemplation of the Form of the Good. The
philosopher who has left the cave will not disire to return to it, but for the sake of
the community as a whole, must do so. The inherent tension is that the philosopher who
knows the good must act in a benevolent manner for the good of the community, but active
practical benevolence is a distraction from contemplation of and return to the
divine.