Hmmm, what do I build and why..... My apologies; this is a long story here, feel free to pass it up.
Hello, I am the Duke and I have a royal case of plasticholism.....
What do I build:
I primarily build all my aircraft and armor in 1/72 scale. This is because of a couple reasons: one is that my mother 'dropped' me down to that scale after building a 1/48 scale aircraft with me and saw that it was a bit complicated for a 6-year-old boy. 1/72 scale was more simplistic and easy for a boy my age to build on his own. The that I stayed with that scale is that a few years later, a new kid moved in and he had a ton of 1/72 scale aircraft. His reason for only building in 1/72 scale: consistency in scale and the ability to size comparison. I had to admit in my influential youth that seeing the consistency in his collection appealed to my sense of order and, well... consistency. Therefore, all my aircraft remained in 1/72 scale. I tried to do the same with ships in 1/600 scale but the ship model manufacturers didn't cooperate. If I wanted a decent Navy then I had to adjust my requirements. Thus, all my battleships and carriers (with the exception of the smaller British and Russian carriers and French and Italian battleships) are in 1/500 to 1/600 scale while all my cruisers, destroyers, frigates and other smaller vessels are all in 1/700 scale. My civilian cargo ships are in whatever scale I can get them in; but still mostly conform to these 'guidelines'.
My armor was also restricted to 1/72 scale (or 1/76 scale when a 1/72 scale version was not available, they were 'close enough' in my young mind) simply because of the same reasoning I had with my aircraft. It was even more appealing when I got to place any of my vehicles next to any of my aircraft to see the size differences; especially when I sat a jeep or small truck next to my C-130 for instance. That just cemented my determination to remain in my chosen scale for these two subjects. Finally, having everything in the same scale made storage and display easier since most of my stuff fit into a small area that gave me plenty of room for new subjects. Except for the larger aircraft; they got hung from the ceiling out of the way of everything except the stray pillow from an occasional pillow fight. Amazingly enough, I didn't lose a single aircraft irreparably damaged from such hazards. And I was right about having the space; it wasn't until I surpassed 1100+ completed models that I started to run out of room to display anything new that completed.
Now:
Why I build what I build.....
I started with aircraft because my first model was an aircraft that I built with my mother. Also, my father was an officer in the United States Air Force and so I got to see dozens of aircraft every time we went on base. They were the coolest things I had ever seen, especially when I got to watch them fly overhead!!
As for why I only build in 1/72 scale; when I was building my models side by side with my childhood friend Emmanuel (and learning how to paint and detail my models from him) I would choose subjects that would (hopefully) be better than what he was working on in a 'fight'. You see, during our first joint build sessions at his desk; we started out talking smack about whether this (insert name of aircraft or ship here) could beat that (insert name of aircraft or ship here). So after a couple models, we decided to see who's collection was stronger and who was the better 'commander'. We started a war game with our own rules to try and balance WWII and modern equipment, and so our build sessions became a sort of 'arms race'. When a second kid moved in down the road with a larger collection than our two combined, which included 1/72 scale armor (he'd gotten most of them from his older brother when the older brother stopped building models) that got me hooked on armor and vehicles just so I could challenge him on the battlefield when he joined our wargames. I also started building helicopters then (still in 1/72 scale) since I was reading about how they were employed in Vietnam at the time; as well as their intended purpose as an anti-tank weapon. I had fun utilizing my helicopters against Jason's armor in our games!
Nowadays when choosing a new subject to build; I am still slightly influenced by that old wargame mentality of trying to build something that would give my 'army', 'navy' and 'air force' an edge against another imagined opponent. Now, however, it is also influenced by finding something that I haven't already built, or something unique to my collection that almost nobody else has. (re: Dora rail gun for example) I am also interested in finding and building as many variants of a subject; or examples of the same vehicle used in different countries' armies, particularly when it comes to ground vehicles. (re: T-72 tanks, Shermans and M-113's for example) Finally, I am also interested in finding models of vehicles that have long and steady service in various militaries that I saw while I was growing up. Not to mention those vehicles and aircraft that were seeing in news footage and books covering the various wars and battles that have happened from Korea to the present. (SCUD B's, Abrams and Bradleys for example), along with other modern vehicles and aircraft. Recently I've been interested in doing many WWI vehicles due to the interesting way they were built back in the day; as well as the uniqueness of many of them. In particular, I love the potential civilian vehicles like the Vauxhall D staff car I'm building now along with the trucks and other vehicles used (e.g. FWD 3-ton truck). In relation to that, I always enjoy doing buildings and bunkers in anything close to the scale of my vehicles so I have something to put on a diorama, layout or photo base.
I also have started building real space (still in 1/72 scale; again for comparison and consistency purposes) and even Sci-Fi because of the influence of Star Wars, Star Trek, Babylon 5 and Firefly. Those shows in their own ways made Sci-Fi fun and believable for me.
I have dabbled in cars; but what makes me choose them are their uniqueness such as the Police Paddy Wagon; Red Baron, 1911 Christie Fire Truck etc. I may do a 'regular' car someday too, but again; it has to be something that appeals to me given the difficulty I have getting a smooth, even coat on the model. Finally, large figures have to appeal to me greatly as well, such as the Polar Light's Headless Horseman and Frazetti's Death Dealer. Otherwise, I don't do them.
Well, now that I have bored you all with my lowly life story I'll leave you all to enjoy everyone else's stories here. Again, my apologies for the long ramble; this was a tough question to answer.