As a Christian, I believe suicide is wrong because I am making a decision that is only God's to make. Years ago, I spent a lot of time hunting through the Bible for a way to justify suicide because I didn't want to go on living. But the more I read, the more I was confronted by how much Jesus had suffered for me and how arrogant it was for me to decide, in the face of so much love for me, that I had the right to opt out of life if I wanted to.
Also, to say that I can't cope with the pain of living any more and then to die in a way that causes so much pain to others is like saying that my feelings are far more important than anyone else's. I know a lot of people say it would be ok for them because no one would miss them, but how do they know? One of my dearest friends killed herself aged 25. For the past 13 years, I've had to deal every day with the pain of her death, of her absence. That's more than half as long as she lived. I wonder if she would have done it if she had known how much her friends were all going to have to suffer for the rest of their lives.
I'm not judging people who have taken that pathway though because I think they are often so pain-filled and confused that they don't know what they are doing. I know someone who killed himself just before Christmas and he left behind a pile of neatly wrapped Christmas presents with instructions on who they were to be delivered to, because he didn't want anyone to be disappointed that they didn't get their present from him. The fact that he could even imagine anyone would care about presents when they were dealing with such a horrific bereavement shows that his mind wasn't really functioning properly.