Who is it that's asking you to justify it? I'd say the statement itself relies on a couple of things. One- that it's a wrong thing to do. (We don't need to justify doing things that are right.) So- then- you have to ask- why do they think it's wrong and- do you agree with them? I doubt it's something you'll be able to change their minds over to be honest. I expect the main reason they don't support it is because it will hurt them so- then it becomes- 'Can you justify hurting me?' I suspect most people's answers would be- 'Because I can't cope with the amount of pain I'm in. To really empathise with that- you'd see that death would be a kind release for me.'
The second thing I think is maybe this underlying belief of theirs that you don't actually want to die. You just want to live under different circumstances. If it's a therapist asking you, or someone that cares about you- I imagine they are trying to locate the things in your life that need 'fixing' so that you are no longer so unhappy that you want to die. Again, I doubt that they'll accept that things are unfixable. They seem to think life is compulsory, so, they never run out of options and platitudes.
It's questionable over whether you'll ever be able to convince others but if it's yourself you are having to justify it to- then I guess you need to ask yourself why. Why do you think it's a wrong thing to do? Maybe it has links to religion? The first time I had suicidal thoughts- aged 10, they frightened me. Because I'd been taught that suicide was wrong and suicides go to hell. It didn't actually take me that long to come to the conclusion that these were other people's beliefs- not mine.
Or- maybe you do actually think there are other options for you. If you are needing to justify suicide to yourself- then- maybe you see it as the less than ideal option. Then, it comes down to whether you can fix your life or whether you can accept what damage suicide could potentially do to loved ones. Again- this is something I personally can't do at the moment. I can't justify suicide to myself at the moment because of how much I think it would hurt my Dad. Really- it depends on your own circumstances.