Gasoline does not stay good forever. After a period of months, up to a couple of years, the processed hydrocarbons break down. The fuel loses power. Moisture and even bacteria build-up. Eventually, gas 'gums-up', becoming gelatinous and totally unusable.
All you dystopian fiction writers, take note. When you see Rick and the gang from The Walking Dead siphoning gas four years after the Zombie Apocalypse, they are sucking up a marginal goo at best. At worst, untreated gas more than a couple of years old will clog a gasoline engine so bad it will kill it. No more runs to town for you, Rick.
There is a solution. Many of you have heard of a gas stabilizer called Sta-Bil. It's OK, and definitely better than nothing. It displaces water, kills microbes and prevents gelling of gas. The main problem is that it is only rated for stabilizing gasoline for a year, and may be effective for two or more (depending on your storage conditions).
A much better fuel stabilizer is called PRI-G. I couldn't find any retail stores carrying it in my area, and had to order on-line. It's rated for two to three years, and they claim if you add a few ounces per gallon every year, your gas will stay good indefinitely (I'd still rotate my fuel stock every three to five years to be safe).
And check your storage containers. I bought a couple of "jerry cans" at a garage sale recently. After filling them and getting them home, I checked and one can had a pin-hole leak. Luckily, my generator had an empty tank and I was able to transfer the contents. A much better scenario than having five gallons of gas slowly seeping onto the floor of my shed. Remember, it's not the gas itself, but the fumes that are volatile. Gas vapor in an enclosed area makes for a big "garage or shed bomb".
If you store gasoline for emergency, don't just "file and forget" it. Treat it, rotate your stock, and for God's sake, check it and store it safely.