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.