The reasons why lawns die are varied and many. Looking at the symptoms will give you clues. Make a list of observations, keeping certain criteria in mind. Note the pattern of dieback -- for example there may be small, round, yellow patches or large yellow areas of lawn. Note the colour of the grass. It may be yellow or straw-coloured, or may have streaks of red. Take note of the actions or events that preceded the dieback. Fertiliser may have just been applied, there may have been a hot spell or a freeze or the neighbour may have just obtained a new dog. Then look at the most common causes of dieback and see how many symptoms correspond.
Too Much or Too Little Water
Lawns need to be watered frequently but, more importantly, they need to be watered deeply. Not watering deeply causes roots to form at the surface, where water is available, and makes lawns subject to stress during dry spells. Too much water makes lawns susceptible to fungal disease, which they are prone to anyway. Generally, lawns need about an inch of water per week during their growing season (summer for warm season grasses, fall and spring for cool season grasses). This can occur naturally through rainfall or supplemental irrigation. Test the amount of water by placing a container in the yard; when it's about an inch deep, it has received adequate water.
The source of chemical damage can be anything from fertilisers to dog urine. Grass that had died because of over-fertilisation often looks dry and greyish green. Salt damage will look like a swath of straw-coloured grass. Animal urine will show up as a patch of dark green grass with a dead spot in the middle. Herbicide drift may kill grasses too, so take care when spraying weeds in flowerbeds, especially on a day that's a little breezy. Unless the weed killer you use is specifically for broadleaved weeds, even a tiny bit can harm a lawn.
Insects will chew on the grass blades or the roots. Grubs are little white C-shaped creatures that eat grass roots, so they reside just under the soil surface. In a severe infestation it's possible to pull up large sections of turf. Cinch bugs eat on the lower sheath of grass blades, sucking the sap and depriving the grass of water and nutrients. Webworms feed on leaves and stems, leaving yellow or brown patches in the lawn, which gradually increase as the feeding area increases. They are about an inch long and are green or brown with dark spots.
Unfortunately, lawns are prone to fungal diseases. Fairy rings are a kind of fungal disease that leave yellow circles in a lawn. Sometimes you will see a ring of mushrooms before the yellow rings appear. Rust will cause the grass to turn light green, then the blades will become spotted with red spores, giving the lawn a slight orange cast. Brown patch fungus causes brown dead spots to form in the lawn, and the patches become increasingly bigger.