Why does New Zealand seem so encouraging of immigration?

I was reading a book about immigration that covers immigration into many individual countries. It said New Zealand was has launched it's own marketing campaign to attract middle class American immigrants. What are the reasons? I checked out their website, and I must say, I've need seen a country provide such a helpful and encouraging web page devoted to immigration.

http://www.emigratenz.org/

I checked out their various lists of desired fields of employment for new immigrants, and the list seemed rather varying and extensive. Why the sudden need for so many different types of professionals?

Part of the reason I'm confused is that my father (who worked abroad in many countries as a professor) said it used to be a difficult country to immigrate to. If he was correct, I'm wondering why this has changed, and what developments lead to this change.

Your father is right about NZ being picky with who is let in, but your impression that many professionals of varied backgrounds are welcome is also correct.
The perceived openness mirrored by the government website merely spells out what every country with an immigration track would also consider desirable; it's just that many other countries haven't got websites as good as that one.
If they did, they would look very much like that also in places like Sweden, Argentina, Canada, etc.
Another reason for the sudden welcome mat for so many more immigrants than before is the fact that NZ is currently experiencing an economic boom - nothing major, but good enough to be able to take people on board, and it takes specialists to run stuff well, everywhere. The professions may seem unrelated, but they actually are connected. Why would they be looking for chefs AND doctors, you may ask? Well, doctors like fine dining, and will spend money on it.
A wealthy society comes in packages.
;-)

yeah! really and have a great day

Because the same as America, Canada, Australia,
They need a population, they need skilled workers like doctors and teachers

that is how America was built... on Immigrants, illegal ones!

Must welcome the Mexican workforce from our laziness.

It is the same as Australia really - need skilled workers.. and also the population isn't rising that fast. If you look at it this way, for every 50 babies born a day - there is around 49 deaths a day ... so the population is not growing that fast.
Also New Zealander's move overseas to work etc....
any country has to have a policy of attracting immigrates and allow so many refugees per year.