Why is an image formed when two light(one to the focus and one to the lens perpendicularly from the top of the object) rays coming from an object intercept?
I am assuming you are talking about either lenses or mirrors. In order for one of these systems to form a real image, all of the light rays coming from a single point on the object must converge at a corresponding point in space. Like @npojo said, there are certain rays coming from the object that are easy to draw by hand in diagrams, so we use those. But to form in image we must have light rays from each point on the object converging at a corresponding point in space.
This applet is fun to play around with this. It shows only 3 rays from different points of the object, but in fact every ray from each point is converging at the corresponding image point.