Why is why the most important question to ask in chess positions?

and what are some reasons when why isn't a good question to ask? 

Let me guess. "Don't ask why" is your number one tip for success in bullet chess?

Sorry don't know where you get the idea that I'm giving a suggestion on know how to succeed in chess. I'm asking why it's important to ask why during your chess game and when it's not a good time to ask this question. 

Here are my candidates:

1. Why did my opponent just play the move he/she did?

If you don't understand this, there may well be a major threat you've overlooked. Also, look for *multiple* reasons, since incidental to that innocent developing queen move to QB2 you may be about to loose your pawn at KR2 by BxKRP+ since the queen may have just backed up the bishop on that diagonal.

2. Is there a tactical shot or winning combination in this position, either against me or for me?

The most important times to ask this are in the opening, where many such tactical moves occur, or when you have tremendous pressure on your opponent, especially in the middle game, when he/she is tied up in knots, whereupon it just takes one stab in the right way to cause his/her structure to shatter.

3. What is the weakness created by my opponents last move(s)?

As one tournament director once told me, if you let your opponent get away with a weak move, it becomes a strong move. Weak moves *must* be punished if you want to survive. Every move has a strength and a weakness, and the way to exploit an unusual-looking move is to analyze it to figure out what weakness it caused.

I suppose all these could be summarized with a single general question like "What things are going on in this position?" but such generality is less useful for pursuing a specific part of analysis. As for when to ask these questions, I believe you need ask only when indicators are present that something special is going on, like high pressure, an unusual-looking move, an opening position with many possibilities, or an endgame with an excessive amount of force (to avoid a stalemate).

is "why am I not seeing any tactical threats?" a viable qusetion when assessing one's position? 

I like that..

Of course, especially when the answer is simply "Because there aren't any." But you have to first check that their aren't any such tactical threats.

Interesting to focus on your opponents last move rather than the position on the board.  I'd worry this could create blind spots as you speculate on what your opponent's plan might be as opposed to what their best move might be.

I think "what" is far more important than "why" in chess.  I don't care about my opponents motivations, rationales or plans, I care about questions like:

• What imbalances exist on the board right now?
• What are the weaknesses and strengths in my position?  In my opponent's?
• What is the best strategy that my opponent could possibly employ against me based on the position on the board right now. 
• What is my best defence to this strategy?
• What is the best strategy I could employ against my opponent based on the position on the board right now?
• What is my opponent's best defence to this strategy.

Play the board, not your opponent.  "Why" focuses to much on the latter, whereas "What" gets to the heart of the matter and makes your assessment objective.

Objectively you're right, TheGrobe, but that assumes a player has perfect analytic skill. For those of us who lack perfect analytic skill, understanding your opponent's plan narrows down the number of possibilities to consider, which means we can assess a position more quickly.

There is a lot written the subject (and recorded) and while there is a lot in common in what to do, each person has their own spin on it. I think it all comes down to evaluation of the position and coming up with a thought process to try and find the truth (or as close the truth as you can get); one that works for you.

I'm really bad at evaulation so I have been trying to read up on it and practice some. I've done one Stokyo study thus far and found that I just don't truly understand how to evaluate a position correctly. I'm trying to assimilate a number of different Author's ideas to find something that works for me.

Dan Heisman's article on Evaluation Criteria is a pretty good one and matches what some other authors suggest. In any position you should evaluate the following 4 things:

1. Material
2. King Safety
3. Total Piece Activity/Mobility
4. Pawn Structure

Some authors will include other things to look at but at my current understanding a lot of those things actually fall under one of those four. For example, GM Melikset Khachiyan in his first video on evaluation and planning talks about seven things:

• Material Balance
• Pawn Structure
• Position of your king
• Development
• Center and Space
• Open Lines
• Concrete Threats

I think the last four listed there, really fall under Mobility and Piece Activity. Silman addresses many of the same points in his How to Reasses Your Chess book.

I also don't think you need to do the evaluation necessarily on every position. It is likely your initial evaluation will present a plan that will hold for a couple of moves at least. Once the position has significantly altered (one of the four criteria has changed significantly) the you will likely need to re-evaluate.

Plans will involve trying to improve at least one of those areas for you or trying to hamper your opponent in at least one. For me, it is still a work in progress, trying to truly understand each idea and how it plays into the evaluation of a position and to start getting better at doing it.

Except you gotta know how to find these imbalances in infinite amount of positions. 

No, you have to know where your time is likely best spent evaluating, and eliminate the lines that are unlikely to bear any fruit.

The only time I ask why is: "why did he play that dumb move... oh, it's actually good, uh-oh."

The rest of the time I'm telling myself a story where the pawn structure is something like the theme and setting and the pieces are the characters.  Meanwhile pattern recognition and calculation takes care of the concrete stuff.

An important habit I suspect many don't do is look at at least 2 different variations and then compare them to see which is better.  I think most people work hard at refining one variation, and if they can get it to pass their bar (wherever their bar may be set) then they more or less instantly play it.

You're evaluating the position to see what the strengths and weaknesses are. That will hopefully guide you to a plan and limit the lines you might have to calculate. Or course, you also have to be able to evaluate the end of the line accurately enough to not make your position worse.

Important questions don't need to always be a "Why"

- What do I need to do in this position?

- When do I castle? When do I attack on the flanks?

- How do I win this "won" king and pawn endgame?

- Where are the good outposts for my knight?

Vot should bee my plan ? [Transylvania accent] 

What should be my plan? [English] 

Based on pawn structures, activity of pieces, king safety, reductions to better endgames and all those pretty nerdy things. 

"Why is why the most important..."

Why is not necessarily the most important, but purpose is one of the 8 fundamental aspects of reasoning and critical thinking. It is so integral and universal, that it is difficult to imagine that it is even possible to reason without it. The others are: 2) Question or issue, 3) Information, 4) Concept, 5) Inference, 6) Implication, 7) Assumption, 8) Point of view or perspective.

If your opponent obviously blunders, asking why while playing OTB during a tournament may not be the best time to ponder it--just refute it and enjoy the victory. Ask why later. 

Well, maybe a better role is to teach, rather than pontificate or intimidate.

I think you and @spod are saying the same thing.