US needs more oil. Hence all the wars. Iraq - oil. Libya - oil. Syria - oil (US occupied all oil deposits in Syria where and extracts it without giving anything back to the people of Syria).
Afghanistan - I'm not sure, but considering that during US occupation, production of heroine in Afghanistan went up orders of magnitude, and many visitors to Afghanistan reported that US guarded poppy fields, probably it was due to drugs. Kill two birds with one stone: get rich from selling drugs, finance other wars and coups with drug money, and flood geopolitical enemies that happen to be located next to Afghanistan borders with the most powerful drug.
The next target will be also rich in oil, although it would be definitely said that it's only because of lack of democracy in the said country.