As I understand it: to keep Iran from the European gas market, to bolster gulf state allies, to maintain the petrodollar and to weaken Russia.

"...strategic concerns, motivated by fear of expanding Iranian influence, impacted Syria primarily in relation to pipeline geopolitics. In 2009... Assad refused to sign a proposed agreement with Qatar that would run a pipeline from the latter's North field, contiguous with Iran's South Pars field, through Saudi Arabia, Jordan, Syria and on to Turkey, with a view to supply European markets - albeit crucially bypassing Russia...
Instead, the following year, Assad pursued negotiations for an alternative $10 billion pipeline plan with Iran, across Iraq to Syria, that would also potentially allow Iran to supply gas to Europe from its South Pars field shared with Qatar...
The Iran-Iraq-Syria pipeline plan was a "direct slap in the face" to Qatar's plans. No wonder Saudi Prince Bandar bin Sultan, in a failed attempt to bribe Russia to switch sides, told President Vladmir Putin that "whatever regime comes after" Assad, it will be "completely" in Saudi Arabia's hands and will "not sign any agreement allowing any Gulf country to transport its gas across Syria to Europe and compete with Russian gas exports", according to diplomatic sources. When Putin refused, the Prince vowed military action."

https://www.theguardian.com/environment/earth-insight/2013/a...

"Don’t let anyone fool you: As we have detailed since 2013, sectarian strife in Syria has been engineered to provide cover for a war for access to oil and gas, and the power and money that come along with it."

http://www.zerohedge.com/news/2015-09-10/competing-gas-pipel...

"An Iran-Iraq-Syria pipeline is unacceptable in the Beltway not only because US vassals lose, but most of all because in currency war terms it would bypass the petrodollar. Iranian gas from South Pars would be traded in an alternative basket of currencies."

http://www.counterpunch.org/2015/12/08/syria-ultimate-pipeli...

"Many have questioned why Russia became involved in the Syrian war but often overlook the fight over natural gas. As Harvard Professor Mitchell A Orenstein and George Romer wrote last month inForeign Affairs, Russia currently supplies Europe with a quarter of the gas it uses for heating, cooking, fuel and other activities.
In fact 80 per cent of the gas that Russian state-controlled company Gazprom produces is sold to Europe, so maintaining this crucial market is very important. But Europe doesn’t like being so reliant on Russia for fuel and has been trying to reduce its dependence. It’s a move that is supported by the United States as it would weaken Russian influence over Europe."

http://www.news.com.au/world/middle-east/is-the-fight-over-a...

"In 1989, Qatar and Iran began to develop the South Pars/North Dome field, which is buried 3,000 meters below the floor of the Persian Gulf. With 51 trillion cubic meters of gas and 50 billion cubic meters of liquid condensates, it is the largest natural gas field in the world. Approximately one-third of its riches lie in Iranian waters and two-thirds in Qatari ones.
Since the discovery, Qatar has invested heavily in liquefied natural gas (LNG) plants and terminals that enable it to ship its gas around the world in tankers. Yet liquefaction and shipping increase total costs and, particularly as gas prices have slipped, Qatari gas has remained easily undercut in European markets by cheaper pipeline gas from Russia and elsewhere. And so, in 2009, Qatar proposed to build a pipeline to send its gas northwest via Saudi Arabia, Jordan, and Syria to Turkey, an investment of billions of dollars up front that would reduce transportation costs over the long term. However, Syrian President Bashar al Assad refused to sign the plan; Russia, which did not want to see its position in European gas markets undermined, put him under intense pressure not to."

http://www.globalresearch.ca/the-secret-stupid-saudi-us-deal...