Why do people sometimes think for a very long time?

Nevertheless, even in known positions, I check to make sure the position is what I think it is. I am fairly sure I try to take more than 60 seconds to make any move, especially the first through fifth moves of any opening. Maybe that's because I am old. Or maybe it's because I remember rash moves when I was younger when I thought I knew what I was doing in the opening. When you have the typical 40 moves in 120 minutes you have plenty of time not to make an error. I have never felt the time was wasted.

Even in positions well known to you and theory, there's still a psychological factor to consider.  How well do you know the different variations for the opening?  Do you want to try out something new or stick with a line you're more comfortable with?  How well do you think your opponent knows the possible positions?  If they're playing a generally positional opening, do you want to try a risky line to throw them out of their comfort zone?  Or is the opposite the case?

All of these questions (and probably more) can come to mind when you're considering moves even as early as 2, or even as early as 1 (Hmmm... An e4 player.  Do I want to play a sicilian or play e5 and hope he doesn't pick an opening I'm not comfortable playing?  Or how about a nice Caro-Kann or French?)

 I agree with that, which is why I said 60 seconds instead of 5 seconds!

I dont see what you could think about pschologically past 60 seconds, and certainly not at the amateur level when you likely have never seen your opponent before. If its Karpov-Kasparov 4, then yeah, maybe there is more psychology there to work through. But at that level alot of it should be done in pre-game preperation anyway.

Bear in mind any long thinking on some question like this could just as easily lead to a wrong answer as a right one, since your knowledge of what is about to happen and why is very limited.

If I am at the crossroads of a known theoretical variation OTB I make a snap decision in a few seconds, trusting that longer thought will just as likely lead to a wrong answer anyway.

Whatever works for you is what works for you.  Just understand that some people are different.  And my suggestions don't even mention the possibility that your opponent might get distracted (as evidenced by the Tal story that an earlier poster mentioned).  I'd actually expect that to happen more at the amateur levels than at the higher levels where people aren't as well trained to avoid such distractions.  I know I certainly get distracted by random thoughts and tidbits of songs more often than I should.

As to not knowing your OTB partners: as with much in chess, it all depends on your position.  Although, in this case, I'm talking about where you play OTB.  I play in around 4-6 tournaments a year, and generally see the same people every time.  There's only probably around 70-100 regular tournament players in my region.  In fact, a couple of my friends here on chess.com are people in my rating group in OTB play.  I've even played one of them enough on here that he knows I'm a Benko Gambit player when he plays d4.  I wouldn't be surprised if he did a little studying up on the Benko before we next play each other OTB.  On the other hand, he's given me a chance to work on some of my anti-sicilian play, enough that I'm comfortable playing a couple of variations that I might not have otherwise had sufficient opportunity to practice.  The next time I play him as white, I may just take a good half hour to decide which variation I want to play.

 You are assuming it worked out well for him, perhaps his results would have improved by better time management/distribution. I seem to recall reading he was frequently in time trouble, which can't be a good thing.

Reschevsky was arguably one of the greatest natural players ever, yet he never actually played for the championship.

Why should Paulsen be bound by how long it took Morphy to move?  If your opponent starts playing at a blitz rate, do you feel obligated to move at his speed?  How do you know that Morphy calculated the move instead of moving by intuition?  It generally takes longer to calculate on defense when you have to check everything, as opposed to attacking where you just have to find one line.

Since opening preparation is now so important, isn't it rare to spend this much time thinking so early? Like, shouldn't they have decided what variation to play in their home preparation?

 Yes. If you are spending boatloads of time on move 3 of a known variation because you are trying to psychoanalyze your opponent, I think you are just fooling yourself that you are accomplishing anything, and giving your opponent a free gift of time trouble for yourself later to boot.

In your comment u didnt mention is he won or lost after thinking so hard for so long.....if he won then all ok coz everything is fair in war and chess....he is playing with your mental patience.

 But taking a long time to think if you lose is condemnable?

Why do people sometimes move too quickly?

 Usually it's because of steroids.

That's the second most likely reason. #1? Aliens.

 It might also be because of their name, as is the case with Usain Bolt.

Taking 30 minutes for the 3rd move in the Caro-Kann is plain bad. No move really loses, so triple-checking everything isn't necessary, furthermore, every tournament player should have a plan available against the Caro-Kann (or any other major opening) beforehand.

The 30 minutes spent should have been spent somewhere in the middle- or endgame when the position is complicated.

That being said, in the past I too have spent several minutes on my first move (or second, but anything more than 5 is useless.

While it may seem that such long consideration reflects deep thought, the reality is different. People may take a long time during a game to consider moves but if they can't get started it's because they haven't thought about anything. For chess mavens, who spend some amount of time thinking about the game away from the board, they've had a lotta time to "look for something new". If they haven't done so, they won't when at the game. They're just haunted by fear of mistakes.

That's the actual main reason for interminable pondering before moving: the fear of making a mistake during what's a freaking game !

That statement isn't meant to reduce the intellectual value of chess (which is deep) but to remind that it is just a game. Those who don't actually make their living at it but who still invest such importance in not making a mistake have, I posit, a psychologic hang-up.

This is one reason I myself almost never play a game online that has greater than a 5 min. per side time limit. Not because I'm so good  but because there's simply too much else to do than sit waiting for someone to spend several minutes mulling a move.

I further suggest that that entire "thinking it over" notion is a bit overblown. The way anyone achieves greater depth of ability isn't only by conscious thought but by familiarity, achieved by total time spend playing. Chess is like other activities: skill is developed by the amount of time spent at the activity multiplied by the accuity applied. The greatest ability is achieved by those to whom the activity becomes almost automatic. It's not achieved by a computer-like mental database of all variations that are runs through---though knowing patterns is the basis for development, there are simply too many variations for any mind to retain them all.

Finally it all comes down to why one plays: for enjoyment, to learn & develop or to avoid making a mistake & revealing one's lack of perfection.

Strong practical and pragmatic players (like Karpov) usually don't take long thinks early in the game. That lost time could be very useful later in the game, especially in a very complicated critical position.

 Great post.

After my opponent plays 1.e4 I usually think for 30 minutes. The position is so complicated at this time. There are so many candidate move to choose from. If I can't find something useful in half an hour I just resign

This sort of thing happens more often, but come on, it's just bad time management. There are of course important decisions to be made after 1.e4 c6 2.d4 d5 but you should make them before the game.