When i started in design it was hardly 9 to 5, it was always more like 8.45am to 6.30pm... but i wouldn't work those hours for any other job. So i think passion is a key factor in this profession. Mind you even after 3 years the hours didn't change much

but the passion remained.
The only time i seriously contemplated moving agencies was when there was no room to move up or develop as a designer, money wasn't the motivator for change, but passion to do more.
This is an industry that demands new ideas every day, commitment to your work, and always learning. People think it's easy, but we [designers] obviously know better now don't we. If i wanted money i'd work in a call centre, less hours for more pay and considerably less stress.
Anyway, what i'm trying to say is, in my opinion, designers from day one don't think about the money, they think about creating... but, i firmly believe that after a good few years, a mortgage, wife and kids, that passion will turn to apathy and money will play a much bigger role. At that point though, if you're not working to the best of your abilities, do you deserve more money?
I'll end it there, time for coffee