Well I can answer by pretty much saying
what I collect, as it's self explanatory:
-Games I had growing up
-Games I liked, but never had (Star Fox, Paper Mario, Sonic CD, etc)
-Games I never tried but always heard I should (A Link to the Past, Perfect Dark, DuckTales NES,...I have Classic Game Room and you guys to thank for these

)
-Gaming oddities, particularly licensed Nintendo or related (SGB Commander, GCN Hori GB Player Controller, Virtual Boy, SupaBoy, GCN Keyboard controller, Wisdom Tree GB games,...)
-Misc things (Sega Nomad, Nintendo Power Issues, Nintendo Mini Classics, Mario Knex mini figures, Nintendo BD&A beanbag characters, WOTC-era Pokemon cards,...)
-I also more or less collect games from these series: Mario & related (Yoshi, Wario, etc), Sonic, Star Wars, Rareware
So I guess it's part nostalgia and part a mixture of other things. There's no real hard and fast rule for what I collect. Strangely enough, I must admit, I don't have a lot of interest in collecting NES games. Currently I have 6 of them. Maybe it's because I've seen ahoy monstrous that library is via the AVGN and others. But more likely it's because I'm 25 and my first real system was SNES, so NES was a little before my time, personally speaking. Same with the Master System. My dad and I played Atari 2600 and 7800 a long time ago, which he sold, and he has a Flashback 2 and several of those plug and play games that we played, but I'm content playing them on my phone or elsewhere (though I may get Mario Bros. and Donkey Kong in cart form).
I currently own and get things for SNES, N64, GCN, Wii, WiiU, GEN, Sega CD, 32X, SAT, GB, GBC, GBA, VB, DS, 3DS, GG, and PC. I own a PSOne, though I don't currently track down games for it yet. Same for Xbox/360. I may get a Dreamcast one day, once I've filled out my recent retro systems like Sega Saturn. I only own one choice version of a system or handheld (not counting upgrades like DS Lite or Sega CDX) with 1 exception - my 2nd GBA, backlit by wiggy.
My collection is pretty modest compared to most on this site, but it's a selection of games, systems and accessories that I enjoy. 85-90% of my collection I play or have played, and the rest are just because I have a little of that completist attitude and have to get Mario is Missing and Sonic Labyrinth.
Some day I'll post my collection, when I feel it's in a presentable state.