I don't think it is all men - both genders do this thing. It is the fear of being alone.
Basically, this man that you are referring to is a key example:
He fell out of love with his wife, in fear of being alone he started a relationship with you while ending it with his wife. When he fell out of love with you he decided to find someone else as your replacement. When he becomes uninterested in her he will do the same thing (just don't be the fall back girl if he chooses to come back to you -- means he couldn't find anyone else). Anyways, it is insecurity and the fear of being alone.
Because they are so dam vain who is going to do their laundry and cook for them.
Because they're afraid to be alone.
I don't like it. I think people should sort out what they want and then have a break and reflect. I hate it when my sister changes partners. Allways the same - bad feeling reaching hysteria and me dragged in and then Mr Right - again! Why she cannot just have a break I do not know - well I do - she cannot be alone. Its not just men anyway.
Of course if the other new person did not play along or even initiate it then it would not happen. I never would want to take someone out of an active relationship or behave like you say yourself.
Never say never I guess - Love is not reasonable after all!
I don't think that it's just a man thing. People in general don't want to be alone, especially when they're used to being part of a pair. So either they saw the end of their current relationship coming and were already mentally "on the market" OR it was the other person who prompted them to end their relationship.
men and women don't want to be alone and if they foresee a break up they get prepared by securing someone else before its over
It is just like changing jobs...better to have a new one before you quit the old one. Personally, i would never to that to a woman...but hey, that is just me.
they are jerkoffs and don't care or got bored because the girl wouldn't give it up to him.
From my own experience in dealing with a significant other whom has exhibited similar traits...I am seeing that it was co-dependency. He was not a fully matured individual within himself.
fear and weakness, ("I like seeing myself wanted and needed when looking into another person eyes"),,, we all have, just to different degrees,,,,
and as responders here have said, self centered and not caring about other people feelings,,,,,, (need a maid quick, what last 10 years, heck hows that going to get my clothes clean, folded and put away, lol)
and closure and time out as suggested here, well, it can be a two edge sword also that some will avoid at all costs to others,,,,,, isolation and depression ,,
but agree with all in that is is not gender selective!
Because men will always need someone to pick up after them, do their washing, housework etc etc lol
Honestly the only reason I could seriously think of is that some people can not handle the thought of dying alone so they always have a back-up :)