I first started eating vegetarian a few years ago, when my wife spent the summer visiting family overseas and I had the opportunity to make more of my own choices. At that time, it was connected with a desire to eat more healthy things: less fat, less processed foods, simpler and more nutritious things. I lost some excess pounds doing it (about 2 lbs per week, over 4 months, which amounted to quite a difference). I felt great, and enjoyed eating more.
When my wife returned, I started eating meat again, although less than before. When we divorced a year ago, I returned to an essentially vegetarian diet. I'm less conscious of reducing fat this time around, so I haven't lost weight like I did before, but that's OK.
It is difficult to have a very different diet from your partner or other family members, at least for me, because food is a symbolic communion, and it doesn't feel right to me to have family members eating disjointly. I'm flexible enough to eat meat occasionally, but I think that one of the things I would look for in a partner now is a willingness to share my vegetarian eating habits.
A friend of mine recently read a book (sorry, I don't recall the title or author) about different approaches to ethics. The idea is that there are basically three different ways of understanding of approaching questions of what we should and shouldn't do.
The first one is purity - this has do to with ethical principles we take as absolutes. The thought of violating them just repels us, and it doesn't really matter whether the violation is large or small. Cannibalism is an example in most human cultures. We just can't accept such a thing as OK, and we're not receptive to counterarguments or mitigating factors.
The second way of approaching ethics is reciprocity - this is where the golden rule and other concepts of fairness in relationships come in. Stealing is wrong for reasons of reciprocity. We wouldn't want others to do it to us, and we can see the practical chaos that would ensue if everyone stole constantly. Unlike purity, reciprocity allows for some flexibility. Most people would say it was OK to steal food for a desperately hungry child, for example.
The third mode is of convention - things we do just because that is the way they are done, and that is what others expect of us. You don't show up for a job interview barefoot and wearing wrinkled clothes not because it is intrinsically repugnant or because it really hurts anyone, but just because that is not how things are done, and you know that others will judge you negatively for it.
For quite a few vegetarians, diet is a matter of purity. This certainly applies to many religious vegetarians (some Hindus or Jains, for example). It also applies to many who have a strong personal conviction that killing animals is simply wrong, immoral and unacceptable.
I respect purity vegetarians. They are living their convictions about something that matters to them, which is something more of us should be doing. But I'm not one myself. My vegetarianism is more a reciprocity issue, although it is religious.
I'm Pagan, and part of my religious sensibility is a feeling of connection with other living things, and of the sacredness of this world and the life that fills it. I'm grateful to the Goddess for my own life and for the life that flows through other beings too. Living things eat other living things in this world. That is not an evil, or a sin. But we as humans have a reflective kind of consciousness, which we may engage or ignore. I choose to be conscious of how I use other living creatures for food. I'm not comfortable with the way most Americans take for granted that animals must die three times each day just to feed them. It's wasteful, indulgent, and spiritually deadening.
I choose to honor life, rather than exploit it. And that draws me away from seeing the slaughter of animals as a primary, or even frequent, process by which to nourish myself. Now, on the infrequent occasions when I do eat meat, I do so with an awareness of the implications of the act, and its spiritual gravity.