Here's why:

* Structure: Ethylene has a double bond between the two carbon atoms.

* Hybridization: The carbon atoms in ethylene are sp2 hybridized. This means that each carbon atom has three sp2 hybrid orbitals and one unhybridized p orbital.

* Bonding: The sp2 orbitals form sigma bonds with the hydrogen atoms and one sigma bond with the other carbon atom. The unhybridized p orbitals overlap to form a pi bond between the two carbon atoms.

* Shape: The three sp2 hybrid orbitals on each carbon atom point towards the corners of an equilateral triangle. This, along with the double bond, creates a planar, triangular structure for the entire molecule.

In summary: The double bond and the sp2 hybridization of the carbon atoms in ethylene create a planar triangular shape.