One is hotter than the other.
A better connection with one over another.
Child bearing hips.
Personality, social standing, looks, money, nicer car...............
I think it's starts with physical attraction...but pretty girls are a dime a dozen so it's personality that will keep someone around...and owning gravity boots and being willing to hang upside down helps and pizza .. And beer.
The reasons are as varied as the amount of men choosing. The answer usually has to do with the maturity level. Younger or less mature guys will often choose the prettier one, or the more popular one. As they grow maturity wise they tend to pick one who fits their personality and lifestyle better.
My initial attraction to my wife was her intelligence.
And then I noticed all the rest of her qualities.
Asking her to marry me was a no-brainer.
because one might be nicer than the other.