I'm guessing somebody could do a whole psychology course (or more) on us collectors. Having been collecting for over 50 years, it is certainly a combination of many things for me. Have many items from when I was a kid that still make me smile with great memories of family members who helped me find stuff for sure. Growing up there was always the challenge and thrill of putting together a complete set of whatever I was chasing, but I was certainly less focused on completing sets as I got older. Nice items for players or teams I really liked will always catch my interest, certainly because of the happy memories they evoke. And to be honest, almost everything I collect or buy now my mind will say "That's cool!" - and having it be something that other people are looking for or would like is part of the equation for sure. I stopped collecting mainstream cards over 30 years ago when the glut came, and manufactured scarcities like 1 of x short printed cards do nothing for me. Finding unusual, scarce, or better yet one of a kind items like production art still gets my heart pumping!
Funny thing is no one in my family collects, I never have friends over to look at stuff, but it still makes me happy to sit on a mountain of stuff from the past 55 years, and when I look at different items I have, my mind still says "That's cool!"