Guppies, like people, don't tend to die "for no reason". Even where bad genes are involved, this isn't likely to be the direct cause of death. Fancy guppies are certainly less hardy than their wild ancestors, but they are still fairly robust fish and shouldn't present any major problems.
First of all, 99.9% of all fish deaths are down to poor water quality or the wrong water chemistry. Guppies need hard, alkaline water. A pH of 7.5-8.0 is essential. If you live in a soft water area, then either mix in some calcareous substrate to the aquarium or filter (such as coral sand) or add a small amount of marine salt, around 3 grammes per litre (assuming your other fish are salt-tolerant).
In terms of nitrites and ammonium, there shouldn't be any. Nitrates are less of an issue, but you really don't want more than 100 mg/litre.
Diet is the other overlooked factor. These fish eat algae and insect larvae. Vegetarian flake food is ideal, and contains plenty of what they need. Regular flake is less than perfect for guppies because it is formulated for predatory fish such as tetras.
Finally, guppies are small, schooling fish. To a guppy, a larger fish like a big barb or gourami is a predator, even if that bigger fish is totally harmless. Guppies do best kept on their own in a group, perhaps mixed with fishes that stay close to the substrate, like Corydoras. Even mollies and swordtails, though often suggested as tankmates, are a bit violent for guppies.
Cheers,
Neale