Here's the interesting thing about people; whatever you love or hate about somebody is a reflection of what you love or hate about yourself.
If somebody thinks you're a loser because you have something or can do something it's usually because they can't do it themselves, and if somebody thinks that what you're doing is awesome it's either because they value it in themselves or they aspire to it. At the end of the day, though, their opinions are irrelevant.
For me, what makes somebody a loser is a total lack of ambition or willingness to pursue their full potential; the people who accept the shit life throws at them and don't even try to make life better for themselves. Again, though, I feel like this because I used to live that way when I was 16-18 and I felt miserable and I despised myself for feeling like my life lacked meaning.
So, yes, what makes somebody a loser is entirely subjective but you also need to keep in mind that your subjectivity is molded and shaped by your culture and various other factors, so, in the United States you'll find that people have a common idea of what a "loser" is because most people are exposed to the same culture; this same notion probably holds true for cultures like Japan, Europe, etc. A loser is somebody who doesn't fit into the cultural norm on the perceived negative side of the spectrum.
-Richard
