Think about that for one second. Wouldn't you rather look more like Jude Law versus a bloated looking professional wrestler in the WWE?
You see, even if you are a little on the slim side you can still look "cool" and hip. You can wear designer clothes. You can look "GQ", etc.
…but if you put on too much muscle on the wrong places you get that “cheesy” meat-head aura...and that is not a place where you want to be.
Most women are repulsed by guys who are "overdone" !
...But these same women love the right amount of muscle.
Some examples of this are Taylor Lautner, Cam Gigandet, Brad Pitt, Will Smith, Dwayne Johnson (now he has slimmed down), Hugh Jackman, etc...
I realize that attracting women isn't the only reason to put on muscle, but it is most likely at least part of the motivation to attaining a nice looking physique.
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