Sure, when you succeed in understanding someone else's legacy codebase, it builds self esteem. When you fail attempting to do that, it does the opposite.
I have inherited projects where I just could never understand what they were thinking well enough to make the service good (not running out of memory and crashing). It was very humbling; I still sometimes wake up in the middle of the night with things I wish I'd tried, but the assumptions of how it all worked was just scattered throughout a bunch of files and the assumptions of everything was just not how I'd do it.