I think this is because people have buried deep in their psyche an image of their ideal mate that they keep looking for over and over, no matter what. Men, in particular, seem to have a dominant stereotype that they find irresistibly attractive. She might be a mysterious doe-eyed brunette. Or perhaps she's a sleek, energetic young blonde.
The latter seems to be John Cleese's preference. Not that long after breaking up with a blonde comedienne who made fun of him in her routines, he has become involved with another young woman of similar appearance. It will be interesting to see if he has more luck this time!