Why was washington against the formation of political parties and participation in political parties?

George Washington was against the formation of and participation in political parties because he truly believed in Unity.

From his perspective Unity, and not Division, was the key for a democratic republic to survive. He really thought that political parties would have negative effects because they would divide and destroy the United States.  

And that's the main reason why throughout his political life until his death, he was sure political parties were not needed because the country could and should function without them.  

because it took place after the war of 1812 had come to an end.