This should be really obvious.
Velma is a lesbian and she's in love with, danger-prone (i.e. suicidal) Daphne. Daphne is straight and has a thing for Fred but, being a good girl from a sexually repressed home where she was molested and became anorexic, the only way she'll let Fred play around is when she's stoned. Shaggy always has good weed to supply Fred who uses it to get laid by Daphne. Velma likes to get stoned too as she's grown bitter and cynical about ever getting Daphne and, in any event, she and Fred find Shaggy amusing and he's got the fat trust fund that allows them to travel the world as professional meddling kids. Fred and Velma have separately had a few awkward sexual encounters with Shaggy, but neither of them will talk about it. They both blame Shaggy's weed. Daphne sees Shaggy as the only man she can really trust and loves him for it, however the only reasons Shaggy hasn't hit on Daphne is because he believes she's out of his league and he doesn't want to lose his best friend Fred whom he wishes he could be just like. What Shaggy doesn't know is that Fred wishes he could be more like Shaggy who just doesn't care about what everyone else thinks of him and seems to take such carefree joy from life.
None of them shave their pubes. It's the 70s. | 
