1.) I was L5, and while there were a ton of great L6s, there were not an insignificant number of ones who were terrible. I think L6 would have been the right level for me; L7 was beyond me at the time. People at higher levels don't really seem to work any harder. That translates to the feeling that I'm getting less pay than I deserve. This is why I ultimately left, and I immediately found a job at L6 equivalency.
2.) My manager pushed me towards promotion. My manager was L6, going for L7, and he had way too many direct reports (more than 20). His plan was to move up and have me fill in the vacuum behind him. Without someone to take on that role he was getting denied his promotion.
3.) I wasn't a Google lifer. I wanted to work for a smaller company where I have more of an impact on the bottom line, and I cared about what I was working on, maybe a startup. Unfortunately I have the three deadly "M"s for startup founders (Marriage, Mortgage, and Munchkins). My plan is to wait until the kids are out (1 "M" down). I could get the house paid off by that point with the extra equity that comes in a promotion (2 "M"s down). And I supported my spouse when she started her own business; she wants to support me in mine (that's all 3 "M"s). TL;DR - A promotion would have allowed me to close out all of my debts, freeing me up to not work at Google.
But I think on top of all of that, you have to get people to want to work on your project. Some projects are fun or have social clout, but come with no chance of promotion (I worked on Memegen for 4 years because it was fun). Some projects suck, but they come with promo opportunities. Projects that suck and don't come with a promise of promo don't get engineers applying to transfer in (this is what killed Reader).
(1) Contribution-driven people build successful company.
(2) Successful company attracts money.
(3) Money attracts social-status-driven people.
(4) Social-status-driven people take over company.
(5) Company ceases to be successful.
Moral of the story: Filter out the social-status-driven people who destroy ALL tribes.
It's always seemed strange to me how singularly obsessed Google culture seems to be with the idea of promotions. All the weird things Google does seem to come down to "that's what gets people promoted". But... why? It doesn't seem like Googlers are systematically underpaid, so why is promotion such a focus?
Maybe this is part of the reason my own stint at Google didn't work out so well.