why was there x-men clip shown at the end of amazing spider man 2 movie?

If you are guessing why there was a trailer/clip of x-men after the amazing spider man movie 2, I have the answer for you!

If you don't understand what I just said, here's a short summary:

On May 2 2014, The Amazing Spider-Man 2 ran in theaters and at the end, after all the credits, there was a clip for X-Men: Days of Future Past. We saw a brief scene for “X-Men: Days of Future Past” featuring Jennifer Lawrence’s character Mystique. 

While post-credit scenes are normally expected by comic super hero movies, this end credits clip is noteworthy because Spider-Man is distributed by Sony and X-Men is by Fox. Traditionally, disparate comic book movie universes don't overlap.

Here's a clip of the end credits that was played.

Well here's the answer we all were looking for:

Years ago, Marc Webb signed a contract to make two movies for Fox. The first was "500 (Days) of Summer" in 2009 and "The Amazing Spider-Man” in 2012. Webb’s negotiations briefly stalled with Sony as he was caught in a tug-of-war with Fox. Eventually, Fox agreed to allow Webb to direct Sony’s “Spider-Man” sequel, but only if Sony would promote its “X-Men” film for free.”

Though this reasoning might disappoint viewers hoping for a Spidy-X-Men movie crossover, it will delight those who enjoy when businessmen make quirky compromises.