For me it’s the creative outlet. Not just in brewing my own recipes or other people’s kits but the whole shebang. I’m kinda nerdy, and brewing also gives me an outlet for my other hobbies. Made my own temperature controller for the fermenter which let me apply my electronics and programming hobbies, and recently 3D Printing. I also have a wiki server for tracking my recipes and brew days.
I love how you can get into brewing with just a pot, a bucket, a bottling wand, capper and some empty bottles, but can go crazy in a hundred different directions as you grow from there. I do have a bit of Gear Acquisition Syndrome, I’m not sure where I’m going next... Maybe a recirculating pump for the mash tun, maybe an electric kettle, so I can do larger boils indoors.
I also love how people say, “You brew BEER?” Or better still, “YOU made this beer” as if they believe I have some magical powers.