The thrill of the hunt. I love the rush you get when you make a discovery. This can be a real discovery of something new in the universe, or simply the thrill of finally understanding some fine point in physics and mathematics that someone else created decades ago. I love going to meetings to talk to people I have not seen in years, and to 'going observing' at some observatory and getting down and dirty with new observations that no one has ever done before in the history of the human race!