Energy Question

Hi everyone,
I had a question about people's energy, and feel free to move this thread to wherever it fits - wasn't sure where to put it exactly.

I've read on the site a little bit about energy vampires, people that connect to your energy and drain it from you? Sometimes I feel as though I'm connecting to people's energies, but how do I know I'm not draining it from them? For example, if I sit and think and stare at a person, it's almost as if my energy is melding into theirs, and based on what they're feeling, I begin to feel that way too. Does that make sense?

I have one person in particular - my physics professor - I've haven't seen anyone this bright and vibrant in such a long time. He has this amazing glow inside him, you can see it in his eyes and face. When I'm sitting in class, I'm in the front row, and I see him up close and connect with his energy or something, and it makes me feel brighter and I can't help but smile along with him. Of course this works in more negative situations if the person is depressed, but it mostly got me thinking I hope I'm not draining his wonderful energy. Any comments would be appreciated!