OK first thing.. you switch diets way too fast. One week is not long enough for your body to adjust and for you to start seeing results. Your body was probably not used to eating meat I gather.. or it could have been eggs?? Your gut needs to build up more bacteria to digest it. This takes time. So its best to slowly increase the amount of foods that you haven't been eating.
Your question is incredibly general. There is no question that humans thrive on animal products, we have for a millenia, and there is compelling evidence that our brains got bigger when we started consuming more meat. They even did bone analysis on our ancestors and found them all to have eaten meat. It's clearly what we have evolved to thrive on. There are also some things like Vit B12 that you can only get from animal products. Animal products contain a lot of good fat and protein without any anti-nutrients that are found in beans. That is really high level.. I suggest reading the primal blueprint