From the perspective of an investor and someone in high-tech during that period, here is my take:
A few high tech companies had made it big (Apple, Microsoft, Dell) and a lot of people were sitting around bemoaning the fact that we all should have realized that computers were going to be huge and invested early in those companies. We all convinced ourselves that we knew it was going to happen (whether we did or not), but for some reason we didn't put our money where our mouth was and now we were grumpy because we could be millionaires already.
In the meantime the whole Internet thing transitioned from being something that only nerds and academics used to a new paradigm for computing. Many of us reasoned that we weren't going to be suckers twice and this time we were getting on that boat before it left for money-land.
So it became fashionable to invest in Internet stocks. Everyone was doing it. It was guaranteed to come up in any conversation at parties or with friends at work. So with all this investment money out there for the Internet's "next big thing" naturally lots of companies popped up to take advantage of the easy money. It got to the point where brokers and Venture capital firms were beating the bushes LOOKING for companies to throw money at and often they didn't scrutinize these company's business plans very well and/or bought into insane growth projections. Frankly, most of the business plans amounted to "We may not make any money off our users, but if we get enough people to sign up that HAS to be valuable, right?"
Problem #2 was that most of these companies weren't run by proven business types, but that didn't matter. It worked for those rag-tag kids at Google, Apple and Microsoft right? Well-heeled business types who know how to build a sustainable business model are so gauche in the new "Internet Economy".
Also, the implicit agenda of most of these new entrepreneurs is
(1) Get enough funding to make the company big enough go public while keeping enough equity to get rich when it does;
(2) Buy a Ferrari;
(3) Repeat with another company.
Now these investors weren't stupid. They knew what was going on and that most of these Internet companies weren't going to be around in a decade. Everyone was just playing the momentum and planned to get out when they saw "the signal" that the whole house of cards was going to fall. At the time we always talked about the fact that these investments were totally playing with monopoly money, but it was addictive. During the peak, at least on paper, my brokerage account was earning more money for me than my day job.
The problem was, that it was all kind of a pyramid scheme. These dot com companies needed a continual supply of new investment because most of them were operating at a loss and some didn't even have a mechanism to make a profit at all, at least not a realistic one. A buddy of mine, for example worked for an IPO bound company that made a freaking web based contact management system. They didn't charge yet, but they would one day turn on the meter and all of those thousands of customers who signed up for a free account would naturally start paying for something the company was actively devaluing by giving it away for free. This company raised more than $100M in venture capital.
So eventually it started to get harder for these companies to continue to raise new money to pay operational costs without showing some kind of ROI. That is, the tried-and-true model for valuing a company started to seep back in and these companies had to admit that the CEO had no clothes. So without money to continue paying for expensive developers and marketing, these companies started to go under. When a few of the big names tumbled, everyone saw that as "the signal" and it was a race to the bank. The rest is history.