Sounds cool - so my personal reasons / motivation would have been 1 and 6:
1 "Because my friends are there" - but this happens after a platform gets critical mass and everyone is joining because "my friends are there"
6 Because you found a community of your niche, with many cool dudes doing your niche stuff very well - photography, jogging, collecting coins, etc. (Reddit is super powerful in this part! They have communities about everything