The most common argument in favor of euthanasia is an appeal to choice. The philosophical presupposition to this argument is the harm principle which was articulated by the philosopher John Stuart Mill. This is the well worn adage that you should be free to do whatever you want as long as you don't harm others. But, it is interesting that Mill himself argued that one could not appeal to the harm principle to sell oneself into slavery. On the surface it might seem this should be allowed. If I am willing to accept slavery to pay for a life saving operation for my wife or child say, then who are you to stop me. However, Mill noted that the purpose of the harm principle was to protect freedom and slavery clearly vitiates the very freedom the principle is supposed to protect.
I think the same case can be made for euthanasia. To kill oneself is to radically, once and for all, put an end to all future choices. It is, indeed, a radical end to ones ability to choose in way which is much more radical than selling oneself into slavery. After all a slave may be freed at some later time. My point, of course, is not that euthanasia is on par with slavery, but rather that euthanasia is in fact an attack upon the very freedom its most ardent promoters intend to defend. Euthanasia does not protect freedom, rather it is the death of freedom.
The same could be said of abortion. The right to control your body is not a license to kill. As I'm fond of saying.