It's a touchy subject, and I'm honestly not sure if it should be legal or not. Nobody should be forced to suffer when they know they're going to die soon anyways. Everyone has to die sometime, why extend it with suffering? There are two main issues I have with euthanasia though.
1. What is considered "terminal" isn't always the same for everyone. Stephen Hawking was diagnosed with ALS in 1963 and was given a life expectancy of two years. He's still alive today. I'm fairly certain that if euthanasia was available at that time, most people with such a debilitating disease would've taken that opportunity to end their own life. The great thing about science is that we always strive to learn something new and improve on things we create. The unfortunate part is that since medicine is also a science, what is today's pharmaceutical pills can be tomorrow's banned drug. I always cringe when I see old ads about doctors recommending cigarettes, or reading about how bloodletting was used to treat disease. But they honestly did not know any better, and although we know more about the world we still don't know everything about our bodies and probably never will.
2. What happens when we find a cure for "terminal" diseases? Do we just shrug and say "oh that's too bad, he didn't have to die?" How would you feel if you euthanized your closest friend/relative, and then learned a week down the line that there was a new cure and he probably would've lived?