Geeeez, I want to change my answer to the same thing that everybody else is saying. Even when it's not the same thing. Everyone's answers here is bang on correct.
So really .. it's the whole dang shabang that's the addiction. The diversity. The new life. The new discoveries. The sense of awe. The equipment. The living things. The information. The hunt for new information. The hunt for Red October ... oh no, wait, not that. The little tinge of feeling good when you've done something to spread the joy. Learning enough about something that you can teach someone else about it.
All of the above. And more.
