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?