Take two parralel line segments.
Lets say AB & CD are parallel . & AB = CD
Now draw a transversal AD .
Now join AC & BD.
We find that traingle ACD is congruent to traingle BDC ( SSS ) .
BY CPCT we find alternate amgles are equal.
This is generalized as a theory . Because even if AB is not equal to CD , you can always construct this and you will get a rectangle with the transversal as its diagonal. In a rectangles the two triangles divided by diagonal are congruent . Thus alternate angles are equal.