why did Peter the Great want to have absolute power?

There must surely be a passage in your textbook that tells
you the answer that your teacher wants.  Otherwise, there would really be no way to know
for sure.  This is because people cannot ever really know what motivates other
people.

Peter the Great was an autocrat.  This means that
he wanted to be able to rule by himself, with no one to check his power.  This is in
keeping with the idea of the Divine Right of Kings.  This idea said that God had given
power to the royal families and that is why they deserved to rule.  This idea was still
common in much of Europe in Peter's time (and it persisted in Russia much longer than in
other parts of Europe).

So my answer would be that he
wanted it because he believed in the Divine Right of Kings.