First impressions are important mainly because too many people put way too much faith in them.
Sayings like, "never judge a book by it's cover," are the ideal. But the saying, "you'll never get a second chance to make a first impression," is the unfortunate reality of our world. Really, though, the saying should be along the lines of "make your first impressions count, because the vast majority of humanity will never take the opportunity to revise the opinions they form from that first encounter."
What can I say, I'm an equal opportunity offender, and I think one of the most memorable mistakes I ever made was while I was still in college. I went to a small, liberal arts college in Southern Maryland called, St. Mary's College (SMC). Even today SMC remains fairly small, but in the mid 1980's, when I attended, it was just 900 people, even smaller than my High School.
Small enough, for sure, so that you could recognize just about every face on campus. And the tradition was (and hopefully still is), that you always greeted others when you crossed them on the way to or from classes. As a matter of fact, you could always tell the newbies from the rest because they never said hello to anyone as they walked by. But usually even the newbies learned to be social and say hello before too long.
So, it was my Freshman year, first semester, and I too had quickly learned to be social and say hello to people while walking around campus. One day, while walking to one of my classes, I crossed paths with another guy who, I was to later learn, was also a Freshman in his first semester. The problem was, when I looked over to smile and say hello, he walked by, his gaze firmly fixed towards the ground, with a look that, to me, said, "don't bother me."
Okay, that was a BAD first impression. But I made the situation worse because, in my head, I immediately wrote him off as someone who was unfriendly. I did what we all do, told myself a few stories about what the guy might be like. After that, when I saw him in the future, I never really gave him a second glance. I had read him, judged him, and moved on with my life. At least, that is, until second semester.