If the pilot light keeps going out, the culprit is probably one of two things, with a third being unlikely but possible:

• Pilot light line flow set too low, or pilot line dirty or obstructed - The pilot light is just a low-flow branch of the gas line that is always on, maintaining the ignition source for when the burner is turned on. The actual flow of gas to the pilot light can usually be adjusted, to compensate for normal variances in system pressure. In your case, this is probably set too low, which means if the overall system pressure drops (perhaps due to added demand from your furnace or gas stove), the pilot light gets starved. Failing that, the valve or pilot light nozzle may be dirty or clogged, obstructing flow more than you want. This is the most likely cause if you only notice this issue with one appliance, while others with pilot lights are working fine.

• Gas Regulator Faulty, Failing or Improperly Adjusted - The meter outside your home has a regulator (the disc part with the cylinder protruding from one side) to "step down" the pressure in the gas main to a proper pressure for your home's appliances. The regulator, like most, basically works by opening and closing very quickly, letting higher-pressure gas into the system until the pressure "downstream" reaches the desired level, before closing off to prevent overpressure. The moving parts in a gas reg see a LOT of use over the reg's rated life, and the one on your meter may be on its last legs. This is normally something that would affect all gas appliances in your home; in addition to the WH pilot going out, you may notice other pilots snuffing, like on your furnace, or you may notice variations in flame height of your stove burners while cooking. In any case, you can call your utility company and ask for a replacement gas regulator; this is a safety feature and part of their service system beyond the gas meter, so maintenance of it is their job.

• High NG demand causing system fluctuations - This is winter, mild though it may be in your area. Furnaces are running, HWHs working overtime, and more people will be using their stoves and ovens during this season than in summertime. As a result, NG usage is at its annual peak. Your area's "gas grid" may be nearing its capacity, which may be causing low pressure on the service side intermittently failing to replenish the downstream pressure of your home's gas system. Ask your neighbors if they've had issues with intermittent low gas pressure; it may not be just you. If it's affecting the entire area, there's not a whole lot you can do unless you've experienced actual monetary damage that you can prove was caused by the service failure. So, check everything else first, especially everything within your home's system (which the gas company isn't liable for).

Here are a few things to consider, from a Reliance water heater Installation Instructions and Use & Care Guide.

Safety Shut-off
This water heater is designed to automatically shut-off in the event of the following:

1. The pilot flame is extinguished for any reason.
2. The water temperature exceeds 195°F (91°C).
3. Excessive build up of dust on the base ring filter or flame arrestor. See Maintenance of your Water Heater Section.
4. The ignition of flammable vapors.

A thermocouple is used to determine if a pilot flame is present, and will shut off the gas supply to the main burner and the pilot if the flame is absent. This unit is also equipped with a thermal switch, designed to shut off the gas supply in the event the water heater has been exposed to flammable vapors (e.g., spilled gasoline), poor combustion caused by a blocked vent or insufficient combustion air. If the thermal switch opens:

1. Check the flame-arrestor for signs of high temperature (blue or black discoloration). See Figure 29.

   
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2. If there are signs of high temperature, inspect your installation for any problems with venting, combustion air supply and for a flammable vapor event (see "Pilot Light Troubleshooting Flow Chart" section). Correct any issues prior to resetting the thermal switch. If you suspect a flammable vapor incident has occurred, do not use this appliance. Immediately call a qualified technician to inspect the appliance. Water heaters subjected to a flammable vapors ignition will require replacement of the entire water heater.

3. Reset the thermal switch by depressing the small button in the center of the thermal switch. Follow the Lighting Instructions found on the front of your water heater

A high temperature limit switch or ECO (Energy Cut Off) in the tank is used to shut off the unit if the water temperature exceeds 195°F (91°C). The ECO is a single-use switch, which, if activated, requires complete replacement of the entire gas control valve/thermostat. If the ECO should activate, the water heater cannot be used until the gas control valve/thermostat is replaced by a qualified technician. Contact your local dealer for service information.

Also make sure you check and clean the base-ring filter.

External Inspection & Cleaning of the Base-Ring Filter

1. At least annually check the base-ring filter (Figure 29) for any dust or debris that may have accumulated on the filter screen. NOTE: If the water heater is located in an area that is subjected to lint and dirt, it may be necessary to check the base-ring filter more frequently.
2. Follow the Lighting Instructions to turn off the water heater and allow it to cool for 10 minutes before attempting to clean the base-ring filter.
3. Use a vacuum cleaner with a hose attachment to remove any dust or debris that may have accumulated on the filter. NOTE: If unable to inspect or clean the base-ring filter, follow the “Cleaning the Combustion Chamber and Flame-arrestor” instructions.
4. After the base-ring filter has been cleaned, follow the Lighting Instructions to return the water heater to service.

The guide also has a pretty good troubleshooting section.

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Make sure the unit has plenty of combustion air. Often after a flood in the basement, folks tend to shuffle things around. If things are stacked too closely to the appliances, the combustion air to appliances can be restricted.