This is a good thread. My initial post-tattoo emotions are euphoric, typically with a side of hunger. But after that wears off (a day or two, often), the depression creeps in. I imagine it's for the same reasons already listed here: I just changed a part of myself and I have to live with and see it forever. So far, this feeling also fades...
But I think part of this has to do with our modern culture, and touches on what @xcom just mentioned. We're all after the absolute best of whatever we can get. With the internet, we now have the ability to research every single decision we make to ensure that it's the absolute best one. I have a hard time purchasing anything over $20 without first reading some anonymous reviews likely written by jackasses. Louis CK does a great bit about this problem of over-research and DVD players: why the fuck do you need the best DVD player? What is that knowledge actually going to get you?!
TLDR - don't worry about the tattoo blues, just keep getting good tattoos!