Casper's mother encouraged his friendliness.In canon where Casper was a deceased person, his mother always taught him to be friendly to people. There are some people who are nice, thus can always expect them to be friendly. Even if no one is being friendly back, his mother taught him not to be bothered by it. His mother's teaching and caring was so strong that when he died, her love was the only thing that kept him from crossing over. That's why he is always searching for friends.
Casper is really an Eldritch Abomination.He only looks like a cross between a teddy bear and a lightbulb to the viewers because that's what the camera shows him as. This is why (in the comics and both cartoon canons) he was a ghost who had never been a human, and he freaks people out without trying. John List.
Poil is the ghost of Gracie Allen.Okay, Gracie was technically still alive when Poil debuted in the comics, but watching the FOX cartoon version, I can't help but see similarities. Also, her boyfriend Spooky, at least in the early stories, had a fondness for cigars, much like George Burns. And, like George and Gracie, Poil's dippiness drives Spooky nuts, though he genuinely cares about her.
Casper always needs new friends because he outlives them.It's been observed that every Casper cartoon starts with Casper friendless, then ends with him finding a friend, and then the next cartoon has him friendless again. If we assume he's been a ghost for a long time, then this is reasonable. His human friends won't be around forever. Unless, like Ferdy the Fox, they come back as ghosts themselves.
Casper is the Morality Pet of Hairy Scarey (from Casper and the Angels).Ghost headquarters assigned Hairy Scarey the job of teaching Casper how to be a threatening ghost in hope that Hairy would be able to make Casper a non-friendly ghost. Instead, Hairy became attached to Casper and it ended up having the opposite effect of what was intended (the events of "Casper's First Christmas" being the turning point), making Hairy become a more friendly ghost who still enjoys scaring people For the Lulz. While he still wants Casper to be scarier like most ghosts are (it is his job to toughen him up), and there are times that he shows some disdain for Casper's friendliness, he really does seem to care about him.