For me, it's taste and a balanced feel to the meal. I like beans. I like broccoli, asparagus, rice, green beans, Brussel's sprouts, potatoes, cabbage - all kinds of vegetables (not that rice or potatoes count as a vegetable!) I love salads. But a dinner of nothing but these "side dishes" if that's what they are being called just doesn't feel balanced to me. I like the taste of meat with the mix. That can be beef, pork, chicken, fish or whatever.
For me, part of a meal has to be something that you like and look forward to. If your meals are nothing but a health lecture sitting there on your plate, I get kind of bored with it. There is a difference between "liking" something and "feeling an enjoyable drive to eat it". I could get by eating only things that I "like" and are good for me. But I would not enjoy meals anywhere near as much. The primary purpose for meals is of course nutrition. But part of a meal - at least for me - is enjoyment too. I don't experience the enjoyment part as strongly with a plate of food sans meat.
