When someone close to you takes their own life it's the single most destructive thing that they can do to you.
You're not only losing somebody that you love unconditionally. You're also left with the overwhelming feeling of how utterly pathetic you are. To not be able to support the person you considered precious enough to have done anything for, the person you would put your own life in jeopardy to protect. This is soul destroying.
The feeling of knowing you weren't there for them when they ultimately needed your support the most is without a doubt the absolute lowest you can ever feel in your life. It doesn't really go away either, ever.
However, the human survival instinct is the most powerful of all instincts we possess. I don't believe it is possible for a person of sound mind to be able to intentionally take their own life. The only way to overcome such a powerful instinct is through mental illness. So for that reason I don't believe it is a selfish act.