why is a triangle angels sum is 180 degree
It is the angle sum property of a triangle which states that sum of all the angles in a triangle is equal to 180°.
There isn't any reason stating why the sum of all angles in a triangle is equal to 180°.
It can be verified by drawing triangles of different kind and thus measuring the each angle and corresponding sum of all the 3 angles in a triangle which will always comes equal to 180°.