For one thing, if the guy says, "Sorry but I don't like you anymore," the girl just as easily says, "who said I ever liked you to begin with." Chances are your guy is uncertain what your feelings are for him.
Girls are very indirect about what they say, speaking through implications and possibilities rather than direct statements. It's not that there is inherently anything wrong with this choice of language system, but the *guy* doesn't even know anything for certain. If he actually knew what the relationship format was or the interests involved, it'd be much easier to be polite about it.
Guys do this all the time in other areas. They cancel contracts with the phone company when they want to switch services. They tell salesmen that it's not what they're looking for. In fact, men are more likely to be direct than women. However, this is when there is a level of directness involved from the other party that allows them to actually gain some concrete knowledge.