I think T hit all the high spots and Hack hit the only thing I could add, especially when Completionism, (which if it isn't a real word, it SHOULD be one.
) is what has driven my collecting for the last 40+ years.
What I like about collecting knives (and knife-related objects) is the variety available in such a simple tool. A knife, in its simplest, is simply a sharp piece of "something" that does basically 1 thing - it cuts stuff. Whether it is a Bowie knife, a bayonet, an axe, a peanut, a Barlow, a trapper, a stockman, a cavalry saber, a gladius, ad infinitum, it just cuts stuff. But oh so many ways of making
a piece of sharp anything.
Some knives are elegant. Some knives are utilitarian to an extreme. Some are beautiful while others are down right butt-ugly. The exact same knife can be both beautiful and ugly. It just depends on who is looking at the knife at the time. I've seen knives you couldn't pay me to own that to another person is the most desired knife in the world. Example - Benchmade and Spyderco knife. I don't like them in general. I gave away the only Spyderco someone gave me. But Spyderco sells a ton of knives so obviously someone likes them, Same with Benchmade. I have 1 only because it was one of 63 knives in an Ebay lot that had a singe fixed blade Western in it that I wanted. The only reason I haven't gotten rid of it is because it has "Prototype" etched in the blade. But Benchmade makes and sells a lot of knives each year.
I started out just getting what I needed. I had less than 20 knives in 1973 - a few worn out folders, a couple of Cub/Boy Scout folders, a Camillus MIL-K camp knife, a few of T's "Trim-Trios" and 1 fixed blade, all given to me by my parents and grandparents, and 1 folder I had personally purchased, a Buck 110 when I graduated from high school in 1973. By the time I graduated from college and was commissioned into the Navy in 1977, I had added a few more fixed blades - a MK1, a MK2, 2 bayonets (a MK6 and a Chassepot), 2 dive knives and a pair of all-stainless steel, no-name, single blade lockbacks from some company in New Jersey that I bought at a Target. They were my EDCs for 4 years of college and much of my active duty time in the Navy - they were slimmer and lighter than my Buck 110 and didn't pocket print during inspections.
Still not into collecting as a passion yet, just a gradual accumulation of different knives as needs arose.
Then, CWO3 Tom Wilson left his copy of Cole, Vol 1, sitting on a table in the Wardroom of my first ship - USS Denver (LPD-9) - I learned that my WW2 MK1 (a Camillus version) and MK2 (Kabar thick pommel) came in different flavors. 4 brands of MK2 with change variations, and nearly a dozen MK1 makers. I started stalking pawn shops, gun shops, garage and estate sales whenever my ship was in port, trying to find something I didn't already have.
Accumulations were slow in the pre-internet days, so I got into swords and bayonets, with help in branching out when I struck on the idea of recruiting some of the sailors on my ships to spend some of their liberty time searching for certain knives/bayonets. I had a standing agreement that I would pay them $10 more than they paid for the items, just bring me a receipt. Gotta remember this was when minimum wage was around $2/hr and sailors didn't even get paid that much. I got a bunch of good stuff I would have never found otherwise, along with a pile of junk. As I explained to a Marine SP once,
"If the guys are looking for a knife or bayonet for Mr. Z, they aren't drinking as much."
As time rolled on, I started adding other types on "sharp/pointy" objects. Axes, hatchets, US and European swords, bayonets of all flavors, dive knives, throwing knives, WW2 machetes/bolos/axes. I see something that catches my eye, research its history and development and decide if I want to add the genre to my pile of sharps. I was called 'The Steel Dragon" by some of the guys in my fire department because they said "You hoard steel like a dragon hoards gold."
When my family asks how many knives I have, I usually say "Not as many as I'll have next year." I will have to admit that my mother has been known to turn to say point blank in front of someone with whom I am talking about knives, "He has a lot of knives. 5 or 6 thousand, isn't it?" just to make me correct her with a real number.
Telling non-knife nuts that I have more than "X,000", whether "X" was/is 1, 2, 3 or 4 leads to glazed looks of incredulousness, shock, horror or fear along with stupid questions such as "Why so many?" "You're kidding me, right?", etc...... so I tend to just tell folks, "More than you have." and leave it at that, unless I really want to mess with their little brains.
When someone asks me why I spent all that money on them, I say, "Well, I'm not doing drugs, getting drunk, smoking stuff or catching loose women, so what's your problem with what I do with my money?" And go back to my hoard of steel.
whenever I asked the Old man about the balloon machine he'd give me 4 quarters and send me to the other machine where I'd get a knife and a bird call.
personality...you can't help being attracted to all that steel. 
