Why do my chargers always stop working?

Every time I buy a new charger for my iPod Touch (generation 4), it works with no issue for a few weeks. Then, out of nowhere, they always start to die out, or give me the "Charging is not supported with this accessory" message, even though it never gave me that before. After a while, they stop working completely and either don't charge at all, or make my iPod screen flash many times until I take the charger out. Even though I always handle them carefully, they keep doing this. I keep having to constantly buy new chargers, and the same thing happens every single time, which is forcing me to shell out a lot of money for new chargers every few weeks. But the strangest part is that, even when they stop working completely for my iPod, they still work perfectly for my parents' iPad. Their iPad charges with no issues at all with the same chargers that don't work for me, even though they're both Apple products.

What's the reasoning behind this? Why does every single charger I buy do good for a few weeks, stop working on my iPod, but still manage to work fine on the iPad? Is there something wrong with the connection? Am I buying the wrong kinds of chargers? What gives?