Pretty well everyone advises to be careful about what kind of dog you choose. Many provide useful guidelines... starting with how much space you have, how much exercise you have time for, how you deal with shedding, etc.
But I would bet that 5 out of 8 dog owners made their decision "on the fly", on impulse or because of opportunity. That was certainly the case with us. I'd never have pictured myself the owner of a Shih Tzu... but by the time we got to our fifth breeder, our patience was wearing thin and it was time to make a decision. Maybe we were lucky but we are happy as can be with our little princess.
So, contra the prevailing wisdom, I'd say yes, it is good to be circumspect, to think about your situation and the kind of dog you are prepared to handle, and by all means, learn about the different breeds, they are often not exactly what you think... but at the end of the day, give yourself license to go with the flow...
Reality is, most of us make enormous accommodations to make our new relationship work, and dogs, despite their many subtle differences, are pretty much alike...
A case in point: a friend of ours in the local dog park encountered a fellow with a delicate little Chihuahua mix he was determined to give away, that minute. It was a wierd situation but something compelled our friend to take the little fellow home. The next week was full of uncertainty. Would he adjust to his new home? What was this little dog all about? Would he get along with their other dog, a Schnauzer-Poodle cross. I admit I had my doubts.
And it's all turned out fine. The Chihuahua has settled in nicely, the two dogs are best buds now and my friend and his kids seem to be happier than ever, part of some bigger, more complete pack.
Maybe it doesn't always turn out so well. Maybe that's why our humane societies are filled with great dogs that just ended up in a wrong situation. Whatever. I think the bottom line is that if you are committed to having a dog, you'll find yourself doing things you never would have thought you could, making accommodations but also laying down your own boundaries... playing the game of give and take that is really what it's all about.