Seeing as everybody's answers are going to be different to varying degrees, I may as well have a go answering myself.
I would add firstly that "spotting" as such isn't my first interest. I place more interest in trying to understand the railway in all its ways. I also enjoy the photography aspect.
1. Why are enthusiasts excited by trains that spew out copious amounts of smoke and fumes when it is clearly bad for the environment?
I'm not particularly bothered about the type of traction. Steam, diesel, electric... I quite like all three to be honest. That said you can better hear when a steam or a diesel loco is working hard than you can with an electric.
2. What is the attraction of steep gradients?
Don't really see such an attraction personally. I was a little curious when I had my first ride over Lickey on a 170 but it turned out we were going downhill anyway!
3. Why do you dwell on the past?
I for one do not. I prefer to stay in the here and now. Not to say I don't find the history element interesting or that I don't like preserved railways, but I actually enjoy modern railways more. I think a lot of enthusiasts prefer the past because they see their favourite period of time as the "golden era" of railways. It is my opinion that such a "golden era" never really exists but if it does, it is one we are moving toward now.
4. What is the point in being hauled by a different loco within the same class?
So that you can travel on as many in the class as possible perhaps? More of a spotting thing I think.
5. What is the attraction to the drivers sounding the horn and doing the up and down movement of ones arm?
I think sounding the horn is sort of a way of acknowledging I'm there a bit like a wave or a 'hello'. I never do any silly gestures. The only way I'll move my hands if in the form of a wave to a passing driver from time to time!
6. What is the attraction of travelling over a length of line for the first time in the dark?
Don't see the attraction personally.
7. Why do you smile at a train when it pulls into a station even when it is a 153 and late?
Now I might be quite a smiley person but I don't just smile at every train that goes past!
8. What do you do will all the photos you take?
I put them on Flickr. I understand not everybody likes endless photos of Electrostars and the like but I seem to have a reasonably sized audience and I think it's good to document the present because one day it'll be the past!
9. Do you all use the terms hellfire and my lordz
Never said such phrases in all of my life!
10. What is the reason for marking off train numbers in a book?
Now this one I've been encouraged to do by my grandfather. I'm also quite a mathematical person and I like excuses to keep lists and spreadsheets etc. I don't know if it's something I'd do if I hadn't been encouraged by my grandfather though!
I can understand being interested in the infrastructure and how it all works and also interested in the engineering of steam trains but the others to be fair I am at a loss what the attraction is. Please don't take this as a wee wee take I see a lot of spotters and am just trying to understand why they do it?
I don't suppose it does make a lot of sense to those who don't partake. It is a little bit irrational I guess. That said, I enjoy photography and that's how I got into railway enthusiasm really. That said, I don't just go out to take numbers, take photos etc. I also find pretty much all other aspects interesting and I do my best to know as much as I can.
As a member of staff in a buffet car said to me the other week, being a train enthusiast has got its perks. Seeing plenty of the country and getting out an about are good things to do. My sense of geography has increased dramatically since I've been interested in railways.
I know it's been mentioned that there's a link to autism. I have no such condition or indeed any other neurological disorders (aside from epilepsy but I've been seizure free since five and it doesn't have that sort of social effect anyway). I do have a tendency to be a little obsessive and to absorb as much information about things as possible though. I'm similar with politics and other interests I have too.
In other words, I prefer to enjoy all aspects of the railway than to just focus on numbers etc.