Haha, wouldn't we need to define the meaning of a cat or a dog or a rock then? I sense a loophole =D!
While I don't really know what to believe about the actual meaning of our existence (there's also the issue of you all not being real; I'm a huge brain-in-a-vat supporter... sort of), I do enjoy the Buddhist/Taoist explanation which involved reincarnation (or was it just Taoist? Forgot):
In essence, there are six incarnations (as in things other than human, not that we only exist six times or we can exist six times at once, though the later sounds awesome) we can take form of. There's a system (karma) that keeps track of our deeds (cause) and issues a punishment/reward (effect). While this may seem like a system that judges things as right/wrong, karma merely returns whats given. Anyway.
The six forms are part of reincarnation, and the goal of it is to reach awakening, which relieves one the need to be a part of the cycle as a result (like Neo not bound by the laws of Matrix, for instance?) Long story short, human is the third one down (organisms non-human is the fourth one down). We're here to reach enlightenment, and... the explanation as to why we're here is vague as usual; basically we're here because we want to be here, once we let go of our desires and... stuff, we'll go back to where we came. Makes more sense if my English was better I guess.