People who suffer from Depression(or other mental illness) should be able to make this choice. If the treatment doesn't work, they should be given options to let themselves go, like the way you put down a loved family pet. No matter how important they are in the lives of others, if they are truly suffering, there should be no reason to force them to live.

Another potential group is people who were born with/suffered great physical disabilities/diseases. This one is a little harder to corner, but there are some diseases that have very low chances of survival, where an assisted suicide can spare a lot of suffering. I don't think cost of operations should be a measure because then we might see people being convinced to commit suicide to save money, which is not ok. The figure of merit here would involve:

chronic pain/discomfort

intensity of disability (quadriplegic, epilepsy, narcolepsy, etc.) this one is hard to quantify, especially because every case is different.

Incurable, deadly diseases. Any sort of disease that has no cure/very high rate of mortality.

The more difficult cases are the people who are perfectly healthy physically, but have suffered great personal losses. The reason that we find it so upsetting when people commit suicide is because it feels like a failing of society to reach out and help people when they are suffering. All it takes is one bad day, and it can happen to anyone. Every time I read a story about someone committing suicide for there reasons, I stop and think about what I would have done if I were in there shoes, and what I would have done to try and help these people if I could.

But at the same time, people need to have friends that are friends with them because they genuinely like the person, not just pity-friends. If a person is completely incapable of socializing or maintaining friendships, what can we do for them?