Toy Envy

Why is it that a toy is more interesting when it is unavailable or currently in use?  And how on earth are you supposed to explain to a nine month old that, no he may not rip said toy out of his brother's hands?  And when said toy does become available, it is no longer appealing?  Do we not do the exact same thing?  I think it must be human nature to want what someone else has or what we can't have.  And when and if we do get what it is we thought we wanted, we no longer want the thing we thought we couldn't live without.  Next time you have an urge to buy something or covet that which you can't have, stop and think about why you think you need that particular thing.  Maybe you really can live without it and get by just fine.