To ask someone "why do you suck so hard?" is a sexual pun that refers to oral sex sarcastically, while generally implying that the person just REALLY emphatically sucks.
You never want to party. Why do you suck so hard?
As if to say:
Why you gotta drag your teeth?
Maaaaan, you deeply SUCK.
You haven't been out with us in ages. Why do you suck so hard?
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