We identify things by both their characteristics and their context.
For example, if something looks like a duck we are tempted to say that it's a duck, and without regard to context that's the most likely explanation.
But then consider the context: If the context doesn't match, we change our assessment accordingly. If it's on top of a mountain, we think it's a rock that resembles a duck. In a store window, we think it's a stuffed-doll resembling a duck. If it's in the MIT swimming pool, we think it's a robot resembling a duck.
Absent any context, Sweden's request for extradition is innocent and benign - how could he possibly refuse such a simple legal request?
But the context surrounding the extradition does not match. There's a number of contextual inconsistencies with the situation, all of which indicate that this is not an extradition, it's something different.
It is abundantly clear that we're not seeing an actual duck. You can argue the probability in various ways, but it's not 100%.
You might next consider "so what?" What's so bad about being extradited to the US?
Consider the risk/reward equation. Julian probably carries around in his head contact information for informants and associates which the US does not know about, and activities of various people which the US would consider evidence of espionage. Once on US soil, it would be nigh impossible to keep this information from the US authorities. He would be forced(*) to give up not only his own freedom, but the freedom of people who put their trust in him. (Not to mention the chilling effect this would have on future whistle-blowers.)
It's likely that the value of this information is so high that even a tiny risk of extradition multiplied by the value potentially lost results in a negative payout. Taking the chance is too risky, it's not a good bet.
... There's no actual evidence for that, and no real reason to believe it.
See previous link, or google for yourself. Plenty of evidence, you are stating an untruth.
(*)Ref: Bradley Manning's treatment