http://www.chess.com/article/view/the-ruy-lopez-by-gm-prasad-and-gm-panchanathan2
I found this to be a very good article on the Ruy Lopez. Read it through slowly, and play through all the moves on the successive boards, taking time to read the comments. There are a few typos that can make it confusing, but if you'll persist, I think you can figure out what the typo was supposed to be.
Of course answering e4 with e5 can lead to complex positions that is the beauty of chess.
But with e5 i directly stake a claim at the center and I only have to see how either i maintain e5 or get d5 in. That is a lot easier to understand (mind you not to execute) than the central strategies in french or sicialian.
Also because of the more stable center piece development is more logical even in unknown positions. It is easier to find the best spot for your pieces
In the sicilian for example development is much harder. What piece should you develop first ? dark squared bishop, light squared bishop, queen side knight ? Choose the wrong plan you will be much worse perhaps even lost.