At first you might think it was because you screwed up your technicals. Then maybe after writing your story on here, the folks will tell you it was because you didn't do a very good job with the. Later some asshat might take it upon himself to inform you it was because you wore a blue with an orange tie instead of a white with a red tie.
And while those reasons all might have had something to do with it, none of them were the ultimate reason why you didn't make the cut.
The reason you were dinged boils down to confidence. Specifically that you weren't confident enough.
It's not enough to download all the interview guides and memorize the answers backwards and forwards. It's not enough to have simply completed an internship or two. You need to know the business inside and out. You need to live, eat, sleep, and drink it.
How do you do this? For starters, don't just do what you are told. Push the limits. If you're an intern, bite off a little more than you can chew. Yeah you might be worked close to death, but you'll learn a ton in the process. If you're not fortunate enough to have internship, then start building your own merger models. There was a guy in theforum a while back trying to write his own full report, and I have mad respect for that. This guy gets it; the best way to truly understand something is by making it yourself from scratch.
Do everything you can to learn everything about the business. If you take this advice to heart and really get it inside and out, the full-time interview will be just a formality. You've already seen and done enough to know that answering questions about an oversimplifiedmodel can't faze you.
When you're confident you can't help but show it. Who cares what coloryou're wearing or if you used the right font on your resume? Of course you'll still be a little nervous going in to the interview, but there's a difference between the standard jitters and a lack of confidence. If it turns out you don't get the job, fuck it. On to the next one. Remember: you want this job, you don't need it.
A word of caution though: always be mindful not to stray into the land of overconfidence. You're still a child and have nothing on the years of experience the guy sitting across the table from you has. Don't try to prove him wrong about some insignificant point or any other crap. You ought to be smart enough to know where the line in the sand is.
In order for others to believe in you, you must first believe in yourself. So pick up your skirt, grab your balls, and let's go make some money.
See a similar discussion: Didn't get the job? How to survive rejection