Chickens, over great periods of time, have been naturally selected in such a way that they are now genetically disposed to cross roads.

                                                       Charles Darwin

Because he was curious, and he had to cross the road to get to the place he wanted to go. Some might say he had no choice, but I would argue that he could have stayed put and chose not to. And then of course, there's the theory that he was pushed. That's also possible, and in that case, he made a good decision - if you are pushed into the middle of the road, it's better to keep walking till you get to the other side where you can pause awhile and then decide what you want to do with yourself.

"To get to the other side"

Whoever made up this question must have had an incredible curiosity towards chickens.

.. Partner otherside of the road.