Question

Why did philip the second want to unite the greek city-states

Answer 1

He just managed to conquer everybody, where the others failed. 
Some of the factors that had an effect on that military operation where his innovations. 
He introduced the notion of professional army in the area and the practice of gaining political and diplomatic influence through marriages. 
Most importantly he managed to secure gold and silver mines in macedonia and thace that allowed him to finance his wars. 
But also the timing was good, the rest Greeks were very weak after the long peloponnesian wars and the plagues of the time. 
Also, since macedonians were also Greek and shared common language and culture, they were able to gain influence in various ways in the other Greek city states, including buying supporters in places like the Greek Delphic council.Source(s):http://www.historyofmacedonia.org/AncientMacedonia/PhilipofMacedon.html