Getting a check every 2 weeks (and other benefits, like Health Insurance) is what motivates me. I spend about 8 hours per day Mon-Fri contributing to open source projects.
(I mean, I don't do it only because of these reasons, I'm a true believer of Open Source, I believe it is the more superior development model for a more civilized age, but it's kinda hard to argue against a paycheck)
There is nothing wrong with getting a paycheck to be able to eat. I spent a roughly 2 year period after college between 2012 and 2014 where I had no job and just worked on OSS exclusively without any compensation beyond nerd credit (and a small credit Google sent me for their online store to buy swag after one of their employees nominated me for an OSS award).
My OSS work turned out to be a great resume builder that opened opportunities for me that far exceeded what I would have had if I had gotten a job as a junior developer straight out of college. The mentorship by the ZFSOnLinux and Gentoo communities was fantastic and helped me mature into a senior level developer rapidly. :)
That said, it is great when you can do worthwhile work and be paid for it. It is a double win.