A friend asked me recently... 'why do people get so upset when their partners leave them?'. Poor guy, he wasn't quite ready for the speech I launched into! :) But I do have a pretty strong opinion on the topic - and since I shared it with one individual, I thought I'd share it with all.
We as human beings love to own things, to possess them. As children, when we are given a toy or a piece of candy, immediately it's "mine". Classic example is when little children go to a birthday party - they're told to hand over the present to the birthday boy and suddenly they burst into tears! This rather nasty little habit never goes away, we just mask it with polite conventions!
So... when we acquire a new spouse or romantic partner, immediately we assume ownership over that person's romantic dimension (and often even more dimensions than that). We now believe that we are the custodians of that person's attractions, desires, and their romantic choices. They belong to us.
Now - this chugs along nicely so long as that person's romantic choices remain in alignment with what we want them to be. BUT - the person falls in love with someone else! They decide they want to be with them! What now? The anger the spurned partner feels is not because they will miss their lover, or because they love them (that would be grief) - the anger is because something that they believed was THEIRS has been ROBBED from them. How dare John take himself away from me.... he was MINE!
Set aside your immediate moral responses on whether it is right or wrong for John to make that choice. Whether he should or should not leave his wife of 20 years, should or should not leave small children, should or should not run away with the office hussy.
Come back to a fundamental truth - right or wrong, a person's choices belong ultimately to them. You may grieve the things those choices result in - you are lonely, your children miss their parent, you are embarassed in front of your friends - but you cannot be angry with a human being for exercising their right to make choices in their own lives.
After all, John has to live with the consequences of his own choices - no one can live those for him. And that unique position gives him that sovereign right.