I've narrowed down getting the feeling of warm glow on my forehead to:

1) Something definitely dependent on other people, most usually when they are very focused or exerting themselves a lot. If I disturb them, or otherwise make them think I'm not 'with' them, the feeling fades.
2) Something that results from listening to powerful songs, usually ambient and entrancing.

It feels so good, I try to give that feeling to others when applicable, my method being just willing them to feel it.