I can mess with my old bikes for untold hours and still never get sick of piecing them together. I do however get sick of messing with the same bike hence I keep adding, bit like a jigsaw, once you've done it it's time for another. Every bike has a story and maybe a little history, some were never popular and others were like rectums with everyone having one 
some start as bare frames and others came in boxes, very few were puchased outright as going concerns and the ones that were I'd purchased new and kept them what seems like forever. The 70's and early 80's for me were gold 
I can't help but try to keep the feeling alive and this is the most fun way I know how 
I'll pay that!
I know I'm repeating a lot of people, but:
1. Taking "junk", and turning it into a rideable bike again. Turning it into a race-worthy bike is even better. There's a whole lot to this: its a statement against consumerism, it's about saving something that was once somebody's pride and joy, knowing that you've preserved a bit of history, and its the simple pleasure of making a seemingly unrelated pile of parts do 'something' useful.
2. Racing something that I've built - proving whether I've done a good job or a bad one.
3. Friends.
4. Racing as hard as you can, but not having any delusions of becoming the next world champ, so the racing is clean.
5. Sensible tracks to race on.
6. Arguing on the internet!
At 32, I don't have a history of racing/riding the older era bikes when they were new (only the later pre-85 bikes start to push the emotional buttons), but than again, I've owned my DT250 since I was 14, so I do have a history there.
6.
