On the large scale of thing, we're only able to witness time through a small window we call our existence, entirely locked from interacting with what happens before or after. It's a bit sad to know that we won't ever be able to witness some historical events, but that's just how life is.

There's no universal reason in living, it's just a wild ride that we witness and try to make sense of. Consider this: your cat's existence, however long or short, contained mostly your presence and affection, and while he's now gone, remember that you gave him the best life you could, filled with you in it. From your point of view, it's a small time window, but for him it was an entire life.

My condolences, and hope you can find some solace by reminiscing the time spent together with him.