First of: judging a person based on the skincolour is totally unacceptable to me, please don't people!
But I do wonder why it is the way it is today. Globally people seem to prefer lighter skins, in India people of the lowest casts have darker skins, but also in Western countries coloured people judge eachother on the darkness of the skin. Is this a cultural thing that spread across the world during a certain period (e.g. Colonialism) or is it almost a human trait (like we prefer symmetrical faces over asymmetrical faces)?
I'm going to talk about China, but this also applies to a number of other countries.
In ancient times and the middle ages, the poor worked in the fields, while the rich were more likely to work inside, as bureaucrats, judges, landlords, and so forth (this also applies to European nobles). Thus, poor people were more likely to get darker skin not naturally, but because they were under the sun all day. So paler skin was a sign of wealth and thus attractiveness. To contrast, in the late 20th century, in the West, tans became seen as more attractice while pale skin became seen as less attractive. Why? Because rich people were more likely to be able to afford foreign holidays.
Now, that was the original reason why pale skin was seen as more attractive in China (and it also applies to many other countries), but in the second half of the 20th century, there was another factor. Popular culture and advertising. Chinese people have, since the "reform and opening up", been exposed to a lot of Western advertising, where pale-skinned women look beautiful, affluent and confident. They haven't been exposed to much Indian or African advertising. This has reinforced the idea that pale=beautiful.
(As an aside: In the Tang and Song dynasties, being fat was considered beautiful, and thin ugly, since being fat of course showed you were prosperous. In the modern day, as obesity becomes a health issue and young people are bombarded with images of thin models, fat is seen as extremely undesirable. And the Chinese have much stricter standards for what is thin than Westerners generally do.)
I talked about China because that's the non-western country with which I'm most familiar, but most of these factors apply to other countries too. White skin has long been associated with wealth because it's a sign you don't have to work outside, and racial biases and economic inequality have reinforced this.
These are the historical reasons: I believe there are also psychological reasons relating to pale skin=youth=female fertility=desirability, but I'm not a psychologist so I can't say much about that.
This would be a good question for the anthropology subs.