It is illegal because the Government in almost every 1st world country cares deeply about their citizens. The well-being of others is a priority in almost every jurisdiction.
if you really cared about someone you would rather let them be euthanized than suffer through a debilitating disease
the only reasons for wanting them to stay alive are all selfish
Since when has a law ever stopped people from doing anything? Especially a law that doesn't affect them after they're already gone.
if you aren't successful you're forced to get medical help
I don't mean picking up a gun and killing yourself just because you're upset that you have no friends or because people call you a friend or whatever. I mean going to a doctor and having something done that will invariably kill you, and without it being painful, because you don't want to put yourself and others in a situation where you will die anyway, after paying for expensive treatments and going through extra emotional trouble