I've been a pescovegetarian (a.k.a. pescatarian, meaning the only fauna I eat is fish/seafood) for around a decade now. Here are the reasons:

1. According to this and many other sources, "There is approximately half a hectare of arable land for each man, woman, and child on Earth. A conventional (meat-based) diet requires one hectare per person. A lacto-ovo (dairy and eggs included) vegetarian diet requires half a hectare per person. ...An animal-centred diet is not only inefficient in land use, it is also highly wasteful of fossil fuels, water, pesticides, and fertiliser and top soil. Production of a kilo of wheat typically requires 220 litres of water, whereas the production of a kilo of beef is likely to require 20,000-50,000 litres of water. " (Yes, I eat eggs and dairy.) The point being, meat consumption is unsustainable as long as the population is so high. Well actually fish consumption is also unsustainable at current levels, but I need the protein.
2. This goes hand in hand with #1: Pollution. "Agriculture is the single greatest source of water pollution in the country." And water isn't the only thing that gets polluted.
3. Disease. "Indiscriminate use of antibiotics may select for drug-resistant pathogens that can affect both human and non-human animals." In other words, it's a miracle we haven't had a major outbreak of some currently untreatable disease.
4. To bother my mother and my girlfriend, since my favorite sentence is "Don't you want to start eating chicken again?"