At this point in continuity, Spider-man has previously been offered membership in the Avengers and declined due to, well, not really enjoying their brand of crazy, if we get right down to it. I have a feeling Thor, unable to accept rejection, just does this once in a while: shows up, demands Spider-man follow him to Avengers mansion, and flies off.
Also what is going on with Thor’s chestplate there, is it just glued to the front of his chest?
[From Avengers #314, 1990.]
"Crap. We need some pinch-hitters. Somebody go get Spider-Man."
"Uh, Tony, didn’t he say he didn’t want to join the club?"
"Sure, but we wouldn’t have asked him to join if we didn’t need him, and let’s be honest, he can’t exactly say he’s not going to help save the planet."
"I think he could, actually."
"Okay, yeah, technically he could, but he’s not very big. Thor can just pick him up and carry him here if he says no.”
"That’s kidnapping, Tony."