Food to the ego.

It's surprising how people can turn out to be immature leeches.
You demonstrate a bit of interest and they respond positively. Then you feed them with that interest and soon they become this mountain of ego ready to blow.
It's like they need someone to stroke their egos and then they become "superior."

Then you turn your back and leave them talking to the wall, and they come back like you're the greatest person to ever walk on earth.