There are many paths to becoming a vegetarian or vegan and each is as unique as the individual who chooses this challenging lifestyle. You may find health issues such as heart or kidney disease spurs you on. Or you may realize the raising and processing of animals has a negative impact on the environment by polluting it and depleting its resources. Like many others, you may decide to forgo animals and/or animal products because of the ethical issue of killing the animals.
Ultimately, whatever the reason is that inspired you to alter your food choices, you have arrived at it after your heart and eyes have been opened to pre-existing truths that you were initially ignorant of. Because of this, you must also be patient with others who have not become enlightened on these issues.
What one eats is a very personal decision, and you cannot persuade someone else to alter it by coercion or by inducing shame. I was once at a meditation retreat where I noticed my Buddhist teacher was wearing leather shoes. Shocked, I asked him why he chose to do this when I knew he had taken a vow not to kill. With not a hint of defensiveness or attempt to present a counter argument, he simply answered, "I'm not ready to give them up yet."
I think this holds true with most people when it comes to their diet. They may take baby steps, giving up meat for one meal a day, or eat meat only once a week. Perhaps, they will at first forgo eating cows, then maybe pigs, then birds, and finally seafood. Whatever the steps they do take, we must always be encouraging and supportive. It does no good for us to get on our high horse and proclaim our superiority. To truly teach our brethren to take the wiser and more compassionate path of eating, we must do it patiently by our own steadfast example.