May've been Bill Gates. In the interview, the guy indicated that he thought that people who didn't finish projects just weren't smart. That it takes a certain IQ level to have the motivation and drive to see things through to the end on your own. I have a relatively low IQ (especially compared to most engineers), so I took offense. To this day, whenever I'm about to sway on a project, I remind myself that successful people will look at me and say it's because I'm stupid, and I go back at it with more fire than when I started.
I don't think this is necessarily true. Many dried out personal projects for me have been caused by that I have solved all technical challenges and only have more of the same to look forward to. That is, solving similar problems I've already solved, and in the case of game making that means endless tweaking and content production - something I just don't enjoy, as it seems. Ultimately though, the potential accuracy of this statement depends on whether this was this guy's personal opinion or whether he had some scientific backing up - I can only speculate.
However, I definitely agree on that it's crucial finding some way of motivating yourself and if that works for you, then you should keep thinking that.