Where the person themselves asks for it I think it should be given. It's their right to be able to choose when they die. When they are in a coma or in a vegetative state or in some other way not in a position to provide consent, it should be proven that euthanasia is 'what they would want' were they capable of consent, ie: through a will or an expressed desire before they were in that state, or an assessment of their disposition through a court looking at this person's life and belongings and family members' testimonies etc. Yes it should be as objective and formal as that. Otherwise it's tantamount to murder.

If the person is in a lot of pain or just doesn't want to live the way he is in right now ,the do it ,My grandfather actually chose to die rather than live the way he was.

I'm for it, it gives you a chance to pick the day instead of waiting for your fatal illness to kill you. Also you shouldn't have to go down suffering, you can choose when you feel you can't do it anymore and end it.