Why are John and Lorraine both happy to be able to have a party in The Pigman?
Basically, John and Lorraine are happy to be able to have a party because it is fun. Both of them come from dysfunctional families, and neither would have been allowed to do such a thing at home. Although they do engage in a good bit of rationalizing when they decide to use Mr. Pignati's empty house for a gathering of their friends, in essence, they just want to have a good time.
John's parents are constantly letting him know what a disappointment he is to them, and he responds by rebelling every chance that he gets, and by pretending he just doesn't care. Lorraine's mother is almost pathologically convinced that without unrelenting supervision, her daughter will go off and do something evil and immoral, particularly involving men. When they have the opportunity to use Mr. Pignati's house for a party, John asserts that the Pigman would want them "to have a few friends over," and would "want to know every little detail" about the event afterwards, "just like he ask(s) about everything (he and Lorraine do) at school." Sadly, John's judgment in organizing the gathering is completely flawed; the people he invites tell lots of other people, and before long, the party gets hopelessly out of hand.
In the aftermath of the debacle, Lorraine reflects,
"We were just playing. Playing. Play was something natural...something which Nature wanted us to do to prepare us for later life."
Because of their family situations, neither John nor Lorraine get many chances to just be themselves and have fun with others their age. Their moments of relief must be secret and stolen, as their parents pretty much disapprove of everything they do. When the two teens receive the opportunity to just let loose and have fun, it is too much for them to resist.