Bb5 in Ruy Lopez opening

Hello,

So this is the Ruy Lopez opening:

e4 e5   Nf3 Nc6   Bb5

I'm not a fan of Bb5, because black can utilize a & b pawns to make the bishop retreat. So black will develop their pawns while white will gain nothing.

Is my concern well-founded or I'm missing something?

@1

"I'm not a fan of Bb5, because black can utilize a & b pawns to make the bishop retreat."
++ That drives the bishop to a better square b3, where it attacks f7. ...a6 and ...b5 also create weaknesses white can attack with a4.

"So black will develop their pawns while white will gain nothing."
++ Pawn moves are no developing moves. You develop pieces, not pawns.
White gains something: an even better bishop at b3 and weaknesses a6-b5 to attack with a4.

"I'm missing something?"
++ Yes. Ruy Lopez is the most played opening at top level and for good reason.

Thank you.

Why is 6. a4 considered a good move?

@3

"Why is 6. a4 considered a good move?"

a4 at some point move 8 or later exposes the weaknesses created by 3...a6 and 6...b5

While a6 and b5 do cause the bishop to retreat, retreating moves are not inherently bad. What you are missing is that these two pawn moves moves create weaknesses on the queenside for Black (all pawn moves create some weakness, even if just a small one) that White can try to exploit by later opening up play on that side with a4. 

It's also worth noting that even if White never plays a4, he doesn't truly lose anything with the "retreating moves." His bishop is very well placed on the a2-g8 diagonal, and White doesn't lose tempo in the sequence, because even though he makes 2 additional moves with the bishop, Black must make 2 pawn moves to induce this. In fact, there is some benefit in the bishop being placed on b3 instead of c4 like it would be in the Italian; it is less vulnerable to pressure in the center, so White can more comfortably expand in that area of the board in ways which are not nearly as viable in the Italian because of the threat that d5 comes with when the bishop is on c4.

If there's anything here you find confusing, feel free to ask and I'd be glad to give an example position or two that show what I mean.