I have seen a lot relationships fall apart like
a pack of cards while only a few survive the
test of time, with the guy remaining eternally
hooked to the girl

Which makes me wonder what is really wrong?
What do a guy see in girl A that makes him want to
leave her for good and what does he see in girl B that
makes him want to marry her?

It is different for people cos some things can be tolerated by some people while some people can't stand it

Different guys...so,different tastes and different preferences in women. But if i should grade an average guy's Scale of Preference in a lady,the following would be his top choice;
1. Good character
2.Beauty
3.Brains(i.e Intelligence)
4.Good 'bedmatic skills'

Different folks with different taste but for most men, it's majorly the tasteful red watermelon in between the legs and the two green apples on the chest.

ebonyking: I have seen a lot relationships fall apart like
a pack of cards while only a few survive the
test of time, with the guy remaining eternally
hooked to the girl

Which makes me wonder what is really wrong?
What do a guy see in girl A that makes him want to
leave her for good and what does he see in girl B that
makes him want to marry her?

We don't need much my dear:
honesty not a liar,pretender and cheater.'but guys cheat also' ehemm must u cheat like them?.
Humble and proper dressing not lady gaga style and avoid been rude which l know even GOD will so much apprieciate it talkless of guys.
Proud and Nagging: deadly virus guys distance themself with jet-speed from you..
Base on my personal observation a lady with these qualities are worth the stress.

Honesty - It's better to be upfront in the beginning even if it's not what you want to hear. A relationship based on lies will be derailed b4 it even gets started.

Loyalty - I need to know that you can stick with me thru thick & thin.

Character - what is she made of? A woman with good character can be the backbone of a relationship.

Independent - Yes I'm the head of the house and I will be the bread winner but can she be self sufficient on her own if it came down to it. What also can she bring to the table. No man wants a needy woman who is always looking for handouts.

Beauty - this might be somewhat petty but if I have to wake up next to her every day for the rest of my life, it would help if she didn't look like a gargoyle.

Kitchen Etiquette - gaddamitt she betta know how to cook. This one is non negotiable.

These are just a few but in my opinion should be every man's core values.