For me, it's knowing that members enjoy the site. I am so happy when members join, start posting, and then on their 3rd or 4th post they will mention something like "I am really enjoying this forum!".
Even better is when they go as far as e-mailing me and thanking me for creating the site.
I also enjoy the friends I have made. My forum is 6 months old, and I am thinking that for the one-year anniversary, we are going to have a local meetup
I don't make any money off of my site, so it certainly isn't that. lol