Why wont your lawn mower stay running more than a couple seconds?

More than likely it is a gas problem. Either the filter is clogged, the gas tank has something in the bottom clogging the outlet, you are out of gas, the carburetor is gummed up or if it is a riding mower it might be the solenoid on the bottom of the carburetor is not working properly. If you haven't used it in a while the chances are it is in the carburetor itself. You can verify it is a gas problem by spraying some carburetor cleaner into the carburetor while it is trying to die and see if it continues to run on the cleaner.