I'm pretty lucky... my brother is a cyclist as well, so he and I chat all the time and did our first big rides together... including our first metric, then our first 3 centuries! It's cool having that in common with a sibling... if we lived closer together I have no doubt I'd ride with him a lot.
My mom is supportive, I have friends I ride with regularly, and have met and got to ride with tprevost from this forum!
My best friend does not ride but IS a marathon walker, so she gets the physical effort and sense of achievement thing... for Xmas last year she had a sterling charm made for my charm bracelet.. it's a silver disc with the number "100" etched into it... she put a note with it saying she was incredibly proud of my accomplishments and this charm was in honor of my century's (I rode my first ones last year!)... now THAT is some cool support huh?
There is a fine line between "hobby" and "mental illness".