Why is the endo approach generally favored in Diels-Alder reactions?

Question

This website says that there's a lower transition state for the endo approach, but it's not obvious why that's the case.
http://en.wikipedia.org/wiki/Diels-Alder_reaction

Answer 1

Take a look at this site. I think it is quite easier to understand here ;)

http://www.ch.ic.ac.uk/motm/porphyrins/introDA.html


Good luck!