Why is the Diels-Alder reaction a syn addition?

It is said that the Diels-Alder reaction is a syn-addition with respect to both the dienophile and the diene. Is there a reason for that!?




<https://www.nyu.edu/classes/tuckerman/adv.chem/lectures/lecture_17/node8.html>


Of course there is a reason. 


[![HOMO of dien and LUMO of dienophile](https://i.stack.imgur.com/tzwko.jpg)](https://i.stack.imgur.com/tzwko.jpg)


Interaction of these HOMO and LUMO is only possible as in the image, in a certain alignment which results in syn addition product. 


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