Just answering your original question "Why do guys "steal" girls from other guys?"
The question was wrong in the first place. If someone took my car without permission or my knowledge- either destroying, selling or keeping it THAT IS STEALING as it was my possession.

Women, however, are not "possessions" and I think you'll find yourself very lonely if you ever claimed exclusive "ownership" of a female.
Many guys claim to be "in love" or "committed" etc, to make up for their possessive behaviour, but that comes across as Needy, Insecure and Jealous (big No No's if you're serious about dating).

As a good starting point I'd get the book "Jealousy The Dangerous Passion by David M Buss" from Amazon to explain why Jealousy exists and even how you can use it in your favour to boost your dating.

Sometimes you have to be Cruel to be Kind especially if you're new to dating

I couldn't really think of another word to use that fitted. Maybe take away? But I think you understand my question tho.
I know the basics of jealously as well as some intermediate stuff. But if the book gets into it in depth I will be certainly interested in getting it. As I do use jealousy as part of my game as I love to use social constructs to game girls.
Which is something I have pick up on from this thread.

In other words if your not boyfriend material to them they are not going to stay around for very long?
I actually started to go out with a girl that wants an open relationship which I am quite down with.

If you believe the studies, ten percent of the population is fathered by someone other than the man who they thought was their father. So infidelity is pretty pronounced as it is. It is each individuals responsibility to remain faithful to their lovers or not.

A better question is how you treat your friends, wings, and others in your direct peer group when it comes to their women? As long as there is a price you personally will pay it becomes a whole different story.

_________________
Artax
Devil's Advocate

"You're only as young as the women you feel."
"I am the Master of my unspoken words, and the Slave to those words that should have remained unsaid."

I've done it (on purpase)
I did it because I could and thought it was fun. When i did it WITH my friend, to my friend. I realized I was an ASS. Even now my nature to steal girls from others (they have proved themselves to be worthey). Any girl, and most people, can be manupulated. But it she did the wrong for you, then she is more likely to do the wrong to you.