I build for several different reasons. My favorite builds are "nostalgia builds'; usually kits I built when I was younger. I think about the models I built as a kid, all the way through college, and I want to rebuild them, but better. I love the smell of the older models when I open the box, even if they've already been opened. If you don't know what I'm talking about, then open an Aurora kit box some time.
I also am trying to build replicas of every aircraft I've flown, and some that I've flown in. As a child, we travelled a lot, and I remember American, Pan Am and TWA 707's being my home away from home. I am trying to build those, as well as the airliners I saw at the time at the airports: Dc-8's, Dc-9's, 727's, Convair 880's and 990's, etc. I have built C-172's and Piper Cherokee-140's, Beech T-34's, and on and on. The one problem with this is that it is virtually impossible to get decals for your N-number or squadron insignia, and definitely not the serial or BuAirNo, and I am not good at making decals, so I just have to get as close as I can.
Less frequently, I build to replicate a machine that I'm especially fond of. For example, I always loved the movie " Bridges at Toko Ri". I am currently building three (AMT, Monogram, Aurora) different 1/48 Panther models with the (spurious I think) Golden Dragon markings.