We'll assume this is not a troll post and you are sincere.
Let's get this out there. In the portfolio of weapons available to mankind, a knife is a poor choice. You have to get very, very close to the opponent, you have to be highly trained and skilled to be effective, and if engaged in any kind of combat with a knife, you may or may not win...but you will absolutely be cut to ribbons...even if you win. Hundreds and hundreds of stitches, loss of the use if fingers and hands from cut nerves and tendons. No, unless you are in the most dire straits imaginable for a human being to be in, your not going to choose a knife as weapon. Now sure, you could walk up to some unsuspecting schmuck in a nightclub or a library or something and stab him...and yes in that context the knife would be a weapon...but that is a one-sided attack.
If you have to defend yourself better to have a striking weapon of some sort...a bat, an axe handle, an axe, an entrenching tool or (maybe technically a knife, maybe not) a machete. Of course noting beats a gun for self defense.
So that sort of takes using the knife a weapon out of the mix for most of us. There are many military knife collectors but remember, since the advent of firearms, the edged weapon has taken a back seat to guns.
History. Arguably the knife is man's second oldest tool (after the club). Somebody, club in hand found a shiny rock, smashed it presumably, found shards with sharp edges which were very useful and the knife was born. It would be a very long time before the axe or the bow were invented.
Metal. Metal has fascinated people since its use became common in about 3000BC...copper. Then about 1750BC Bronze.
Iron. Meteoric iron has been used for a long time but smelting became common about maybe 1000BC or a little later.
Steel. Steel was presumably not invented but rather discovered as a state of iron. An alloy of iron. I think that a date for real steel has not been pinpointed but sometime before the time of Christ...let's call it 500BC or so and let's call it India...most common best guess.
Steel is a fascinating subject. There is great skill in making it. There is great skill in heat treating it. There is great skill in fashioning it into a tool. There is a tremendous variety of steel that would take a lifetime to learn. With carbides or without, big carbides or small, and the benefits and trade-off to everything. The knife is arguably the most pure object wrought in steel...no moving parts, geometry is key, balance is key, heat treatment is crucial, alloy is key. Minute variations in any of these tremendously effect the performance of the blade.
History again. Just about every culture has a traditional knife. Japanese culture is obvious. Finland has the Puukko. Nepal has the Khukuri. Texas has the bowie knife...the list just goes on and on.
Accessability. There are so many knifes, made in so many configurations it is just fascinating. Some people focus on one, others are all over the place.
Hobby. A whole hobby world surrounds the knife. Making sheaths. Making knives. Putting handles on existing blades. Sharpening knives is a skill, hobby, and trade unto itself.
Function. I don't know how anybody gets by without one. Every try to get the shrink-wrap off a bottle of Lea and Perrin? Ever try to open a CD jewel box? Wouldn't be much of an issue...if you had a knife. You needn't have a big Spyderco Military for such a thing. My grandfather ran a farm and raised a family on nothing more that a little, probably cheap, pocket knife with two tiny blades. Did everything from peel apples to clean fish and game.
So in short, What's not to like about the subject of knives?
