The more I thought about it the more I started to agree. I kept asking myself why do I collect? What is the purpose of buying all of these cards. How many auto's or GU pieces of the same player do I really need (or want not need). I guess the answer for me is that I have been collecting cards for 30+ years, I collect more from habit then anything. I am not sure that I even enjoy the card as much as I do the chase of the card.
Hobbies are supposed to be a break from the daily grind. But looking at the questions you posed above, well, they were ones I asked myself when I was debating getting out of hockey cards. Ultimately I did, but I had to think about it a lot. A lot. And I seriously believe that when you start thinking about it in terms of $$$ it's time to rethink why you collect anyway. Once you start feeling buyer's remorse every time you open a bubble mailer, something has got to change.
I feel pretty confident that I had one of the better Jagr collections out there. And I think in retrospect, especially for the last year or so of my hockey collecting career, I collected for two reasons--to maintain the collection and to hoard and keep them from my nemeses on eBay.

. I didn't necessarily have a "gotta have" list, but I found myself raising the ceiling on what I was willing to spend on a card. And I was hitting close to that ceiling with more and more frequency.
To this day I have no idea why I stuck with it as long as I did. Then I think back to all the other things I have collected over the years--Star Wars figures, comics, even beer tap handles. I look at them, then they find their way into a box somewhere. At least comics are a little more involved--you gotta read it, there is some continuity in the story arcs, but I'm not sure that I have ever really re-read anything (aside from things I read as a kid, then more recently).
So why?
It's a great question.
There are actually several semi-scholarly books on collecting, the psychology behind it and whatnot. I've read a couple of them, and none of them really fit me. It just goes to show you that there is no cookie cutter reason, no template for why we collect.