The Avengers were formed as part of a government organization to create a team of superheroes that would work LOOSELY together to protect the United States and the world from any threat too large to be handled by one superhero alone. For the most part these heroes continue to work on their own, but band together when needed. I'm not sure how that sounds silly.

For this movie, Loki has otherworldly allies and as such is too much for Thor to handle by himself - or anyone else for that matter, so this is why they band together. Also note that this isn't by happenstance or sudden at all: if you've watched the movies of each individual hero and waited until after the credits, each one has Nick Fury (Sam Jackson) appearing to talk to the heroes about the "Avenger initiative." This team has been in planning for over four years in Marvel-time, for the reasons listed in my first paragraph.

Now. If you do not like the idea of people who would otherwise not work together having difficulty working together, and becoming a team with a somewhat happy ending...I don't know what to tell you. Should they work together instantly, these near-deific beings that aren't used to being challenged in any meaningful way?