Such a simple question, and I know there will so many different answers.
For me, it is a simple thing. I can't stand the site of blood and never cared for the taste of it. And now that my health is a little off, my meds would prevent me from letting anyone feed off me anyway.
I get such a high though when I feed off energy. Certain types of music can be invigorating as well. I honestly can't say when I first figured out what I am, since I can remember energy feeding even back when I was really young.
What type of what? Canned fruit? Animal? I see that you're heading for a point. Please be more specific and then this will turn into a better conversational forum thread.