You mean, passionate about an area of vet med/future career? It's probably a little different for everyone, but I had to spend 6 months working in a small animal clinic before I realized that life wasn't for me. I liked biology classes at school, but didn't fall in love with any of it until I became a TA and was interacting with the material and lab experiments in different ways. That's how it goes for me - something sounds interesting, I do a lot of book research, it still sounds appealing, I get involved, and after some amount of time I'm either still really intrigued or I realize I don't love it as much as I expected to. Sometimes it's the circumstance (if I'm working/interacting with really awesome people, you can bet I'll be more excited about what I'm doing). I think time is a really big thing. If you're there every day for 6 months and you still look forward to it, I'd say that's passion. But that's just me.