It is one thing to meet, get to know, and fall in love with someone in your town and then for some reason (work, tragedy, military deployment) that person has to move to a distant location. In that case, you already know the person and can make a fairly reasonable decision on whether you will stay together and eventually be together in the same town again.
When the person already lives in another city, it is almost impossible to really know how you feel about this person because you won’t be living with them and interacting with them in person on a day to day basis. There are a lot of things that are not “seen” or “felt” or “realized” about people and their habits and personality unless you interact with them in person on a regular day by day basis. Long distance creates all sorts of illusions. When you interact with people in the flesh you see them at their best, but also at their worst and everything in between.
If you only interact on the phone or online or even by skyping, you will most likely only be seeing the best of the other person. There’s a bunch of stuff (that may or may not be deal breakers for you) that cannot be experienced unless the person is actually with you. Things like appetites (for food, sex, and crazy unknown things) preferences for things like music, temperature of the room, the bed, how often the dishes are done, if they are a neatnik or a slob, how often they like to hang out with other friends or their family, if they want a pet and if they will take care of it or leave it up to you, if they have a laugh that will drive you crazy if you had to hear it everyday or if they spit in public or hock things up or make weird throat clearing sounds or constantly blow their nose, or leave dirty icky things where you have to pick them up, or can’t cook, or don’t like the same things as you, or don’t like your friends or your family or keep a different schedule or prefer a lot of alone time, or prefer to have big parties in their house or don’t keep the bathroom or kitchen clean. This is a pretty short list of things that are important if you plan to ever live together or get married or even live in the same city. But unless you already know these things about a person, it’s almost impossible to know these things over a long distance relationship.
Someone usually gets hurt way more than the other person. Just read some of the long distance romance gone bad threads on fluther.