First of all, I realize I'm a little behind on this topic, but I thought I'd say something.
How do you account for animals killing and eating other animals? I'm no expert on animal anatomy, but I imagine animals could probably get by without eating meat too; they probably don't NEED it either, but they still kill. It certainly isn't greed either; although I think animals could probably be greedy, it's fairly obvious that certain animals are designed to kill, and it's fairly obvious that other animals are designed to avoid being killed, which, once again, proves that killing animals is natural and to be expected.
It is also scientifically accepted that human beings are omnivores; we're designed to eat plants and animals.
The only other thing I've got for now is purely opinion: I don't recall ever coming across a vegetarian who was particularly happy, but I certainly cannot speak for them all.