Different animals live different lengths of time depending upon their evolutionary history.
Animals which are highly predated (that is, animals which tend to get picked off by predators reasonably often, such as mice), have a small amount of time before they reproduce. Hence there was an evolutionary pressure to have short life cycles – those animals which had genes which coded for a late sexual maturity, didn’t get a chance to reproduce – because they died!
This theory also applies to cute hamsters – which live a short amount of time because there was an evolutionary pressure towards short reproductive cycles.
That’s a very interesting question. Generally speaking, the smaller an animal is, the shorter its lifespan will be (of course, like every rule, there are exceptions to this). Why should be isn’t entirely clear. One possibility is that small animals live at a faster pace (for example their hearts beat faster), which may mean that their bodies get ‘worn out’ faster.
Well, some scientists have worked out that the all mammal hearts have about the same number of heart beats, about a billion in one lifetime. A hamsters heart beats very quickly, and an elephants beats very slowly and hamsters don’t live very long but elephants do.
Generally, animals live as long as is needed to reproduce and have more offspring. Take the mayfly for example. The adult, after emerging from the larva, lives anything between half an hour and a day. However, that is long enough for it to breed, and it dies right after that. Even if it didn’t die that soon, it would get eaten very quickly because its a clumsy thing, so instead of investing its energy into living a long time (longevity), it invests it energy in reproducing as much as possible (fecundity).