Full disclosure: I work at Dropbox. Dropbox is a company that values mentorship. It is something that is explicitly called out and rewarded in performance evaluations, and it's one of the reasons I chose to accept the job offer from Dropbox. I have been lucky enough to work at a place that pushed me to be a mentor, and another place where I got to influence culture to value mentorship, and I have seen junior developers who barely had any idea what they were doing and needed lots and lots of guidance, grow into some of the most amazing and capable engineers I've ever worked with.