Saying goodbye to pets

Today I have to say goodbye to my cat.  This cat has been around since University, and was the first pet I owned.  I am really sad to have to say goodbye to her.  But, keeping her going isnt fair either.  She is not as happy, struggles to jump where she wants to be, seems lost sometimes and most annoyingly, is becoming incontinent.  Lord, I hope when I get to the same point in my life that it will be legal to take me out of the picture gracefully and painlessly.  Why is it that we recognize the dignity of death in not allowing a well loved pet to suffer, yet it remains a crime to allow someone to choose to 'check out' when illness has become to much to bear?