Those who find him likeable obviously haven’t seen him on the campaign trail. He’s one angry, nasty piece of work.
His likeability is a function primarily of two things;
1. Whites who think it would just be wrong, and probably racist to think ill of Obama.
2. Overwhelming media and pop culture bias. The image of Obama is that of a brilliant, cool, historic, transformation figure. That is the narrative the media and Hollywood push. Just listen to the late night jokes; Republican jokes are much more mean-spirited. Romney is an aloof, uncaring aristocrat. Bush was an idiot (and he actually was in many ways, but not the way the media portrayed it). Republicans hate women. Republicans hate blacks. Republicans hate hispanics, and so on and so on. The jokes directed at Obama are much more tepid, and never about his unassailable character.
A Republican with the same personality, speaking skills, and record as Obama would have been labeled arrogant, sneaky, and a total incompetent failure.