My passions. I only create forums based on my passions with the exception of Talk Whatever. I didn't want to get into the general forum niche but wanted to test out vBulletin 6 and just made a general forum to see what the software was like. It started growing on me, and after converting it to XenForo (VB6 isn't that great right now), I decided to keep it.
Otherwise, I create forums about my biggest hobbies, interests, and passions in life so that I never get sick of talking about them.
A few forums have moderators, but I don't give the forum to them to run it. I think the forum owner should run the forum. If not, why the hell is he or she running a forum in the first place?
I created my first open forum for making money, and interestingly I managed to make some money. These days I am running forums just for the sake of passion
If there's a topic I'm passionate about I'll create a forum about it. I think having a forum is way more organized than any other method of having a community. I'll admit some days I am tired and don't have time to log into my forums but I always make sure to log in the next day and make a few posts.
Exactly as Shawn mentioned in the second post for the love of the subject, there always has to be a personal involvment and connection if you want it to work.
As far as just ProBoards software goes, still after all these years it's nothing but general forums and RP forums that have the most interest and are most popular. Right from the beginning I've made communities on things I have a personal interest in. Back in 2002 I started with an all-round sci-fi forum that ran for a couple of years. (Most foced on either Trek or Star Wars as a main theme). I've got my Transformers forum still, I've had a Wildlife forum and a couple that haven't worked out an 1980s forum and a film forum in 2020, but all of them I've had a personal connection and interest with.
I've had mods help run my Transformers forum in the past when it was a lot more active but I still remained paramount involved and let them or rely on them to run it. Outside of the forums I have plenty of interests, gaming, photoshop, films, plenty to keep me occupied when not in the mood.
Ever since I saw people creating JavaScript code for free forum hosts, I wanted to do the same. That's where my passion lies. I enjoy creating and supporting coding-related work. It's one of the few things I'm decent at in this world, and it benefits other people in the process.