It's been an up-and-down year for Clean Energy Fuels (Nasdaq: CLNE) as investors try to determine what the company's future holds. The company continues to grow but losses are also mounting, and after a $31.9 million loss in the first quarter of this year, the stock has begun to sink. The falling price of oil over the same time hasn't helped matters either.

As bad as things have been recently, there are still a number of reasons to think Clean Energy Fuels has a bright future. Below are my top three reasons for investors to buy the stock for the long term.

Natural gas advantages over oil
The oil market is a global behemoth that rises and falls with the economic news of the day and whatever the turmoil of the week is in the Middle East. When Iran threatened to close the Strait of Hormuz, the market became scared that the supply of the world's oil could be in danger and prices rose, just one example of the impact geopolitical risks have on oil. Since much of the world's oil supply still comes from the Middle East, the constant dangers there are dangers to the world's oil supply.

Natural gas is a very different market that is largely regional in nature. As the production of natural gas rose over the last decade, there wasn't anywhere for the supply to go internationally, so prices fell. There will be little to change that dynamic until Cheniere Energy (NYSE: LNG) opens its export facility, and even that impact is still years off and the global trade of CNG or LNG is nowhere near the oil market.

All of this means that the price of natural gas will likely remain steady and relatively low as fracking continues to expand and costs come down. The reliable supply will be attractive to businesses, especially if oil creeps back to $100 per barrel, giving a good financial alternative to short- and long-haul fleets.

Long-haul market upside
Fellow Fool Jeremy Bowman recently pointed out that one of the downsides for Clean Energy Fuels is that the development of the natural gas fuel market has so far been focused on short-haul trucks and buses. This has limited upside for Clean Energy Fuels because the network is relatively small and the number of vehicles using each fueling station is limited. But as the company completes America's Natural Gas Highway, the long-haul market will open up and the upside become much higher.

It will be the long-haul market that will determine Clean Energy Fuels' success, and there are powerful companies investing in its future. Clean Energy Fuels is partnering with Flying J and Chesapeake Energy (NYSE: CHK) to expand its network of stations. Chesapeake Energy and Valero Energy (NYSE: VLO) are building stations that will expand the network as well, making natural gas fuel more attractive to both fleets and consumers.

Progress is being made in adding fleets to the long-haul market, and Clean Energy Fuels recently added five major trucking operators to its list of customers. Its natural-gas-powered trucks will operate around the country and will use the expanded Natural Gas Highway.

Westport Innovations (Nasdaq: WPRT) is also providing technology to some of the biggest names in truck manufacturing, increasing the supply of trucks available to fleet owners. Volvo, Cummins, and Navistar have all partnered with the company in the trucking market while Westport is also working with General Motors, Ford, and Caterpillar to expand other product offerings.

Leveraged upside
As more natural gas vehicles reach the road, the upside increases for Clean Energy Fuels. Right now the company is in the midst of an expansion phase that requires capital expenditures to build the infrastructure necessary for America's Natural Gas Highway. As more vehicles begin using this infrastructure, Clean Energy Fuels will generate more cash flow, leveraged by each additional natural gas vehicle on the road.

Of course, this requires a critical mass of vehicles to be profitable, but the momentum is quickly building for the company. First-quarter gallons delivered rose 23% from a year ago and more trucking fleets are adding natural gas in the current quarter.

This leveraged upside is a positive for investors, but it's also the company's biggest risk, so it's a double-edged sword. If the company doesn't reach the critical mass necessary to break even, it will continue to post losses.

Foolish bottom line
This is a high-risk stock, but if you think that natural gas will begin to play a larger role in our energy future than it currently does, then Clean Energy Fuels fills an important role. Trucking fleets are adding natural gas, and this is one of the best ways to take advantage of the trend.

But this isn't the only company that is benefiting from the low price of natural gas; so is the drilling market. We've highlighted one stock that will be a winner, no matter the energy source, in our free report: "The Only Energy Stock You'll Ever Need." Click here for your copy.