Q:

Why does the Puppet Master want to merge with Major?

In Ghost in the Shell (1995), there appears to be 3 intelligent forms

Type 1: a completely organic person. Example: section 9's chief
Type 2: a brain from a type 1 + some codes + a bio-mechanical body; the first 2 terms of the sum constitute the "ghost" part whereas the body constitutes the "shell" part. Example: Major
Type 3: an AI program that isn't tied down to a physical humanoid body. Example: the Puppet Master

Question: why does the Puppet Master (a Type 3) want to merge with the ghost of a Type 2 so that it can share the shell with the ghost? 
The movie explains something about the desire to reproduce and to die but it's not clear to me. In what senses can a type 2 reproduce or die and how does this satisfy the need of the Puppet Master? Perhaps my categorizing of intelligence into 3 forms above is completely wrong so please correct me on that as well if you so choose.

A:

The Puppet Master was a computer program who became sentient. Ever since PM kept itself hidden and observed the world and humanity and questioned its own existence. It wanted to be alive and not just a computer. While it defintely had the faculty of intelligence and sentience, it still couldn't call itself alive. Using its own words:

The Puppet Master: I called myself a life-form but l am still far from complete. For some reason, my system lacks the basic life processes of either death or the ability to leave behind offspring. 

The ability to reproduce in particular was important to PM. Because it felt vulnerable to extinction since there was only one of its kind. And it couldn't simply copy itself, since the copy would inherit its exact weaknesses and be equally vulnerable. It needed the ability to reproduce genetically diverse offspring to increase the chances of its new species to survive. 

The Puppet Master: A copy is merely a copy. There's the possibility a single virus could utterly destroy me. A mere copy doesn't offer variety or individuality. To exist, to reach equilibrium, life seeks to multiply and vary constantly, at times giving up its life. 

To attain genetic diversity it needed a second set of genes/attributes. The Major presented that opportunity, being half human/half machine. She would be able to interface with both machines and humans, giving PM a wide variety of genetic material to use for its reproduction.

The Puppet Master: I can't leave any genes or children behind. After the merging, you will bear my offspring into the net itself. Just as humans pass on their genetic structure. 

