I've been playing the game daily for at least 3 years now, probably 4.
When I first began, I wanted faster, sexier cars. I worked HARD to unlock new cars, and thoroughly enjoyed the process and rewards.
After I'd unlocked them all, my next goal became 100% completion, which I eventually achieved with lots of help from this wonderful forum. With each update, I raced as hard & fast as I could to reach 100% again, which I managed 4 times in a row. Then the dreaded 2nd NASCAR update happened, which made me throw in the towel. It was also around that time when I noticed the WTTT offered Gold rewards for groups A, B, & C. "Let's give it a try", I thought. Gold is good. Gold is a necessary evil.
Without any real trouble (except a quick-but-harsh learning process) I found myself comfortably in group B each week. I also noticed that this required real skill, and really made me really pay attention to the "racing" element of the game that I'd largely ignored until now. It made me acutely aware of the physics aspect, and pushed me to improve my skills.
This is probably the only reason I've carried on playing for so long. I only have 1 real-life friend who plays RR3 as seriously as I do, but he's not fussed about getting fast TT results. On a similar note, I'm not fussed about series completion anymore. I mainly complete a series to open up the Time Trials these days, and only if I like the cars that are involved. If a new car becomes available in an update, I only bother winning it if I think it'll be needed in a WTTT at some point in the future. I will occasionally complete a series if I'm in dire need of Gold, but I do this under duress; I don't really enjoy the process. My thumbs really feel the stress of racing too much these days, too.
I know I will carry on playing until I reach group A in the WTTT (never managed it yet, but I know I'm capable), and I know I will carry on after that to try to maintain a group A result each week. 300G is a fantastic reward, and definitely one motive to keep trying, but in honesty, I mainly want to be in the top 1% of racers. The only thing that will stop me racing is if my thumbs can't cope any longer.
So, why do YOU keep on racing?