Our bodies aren't ours to use as we want; they are on loan by God for our earthly life. It's where God's spirit lives. Even if we don't believe, there's still a bit of God within every one of us. We are supposed to look after our bodies and not use them in ways God forbids. And that doesn't only apply to sex. We shouldn't make ourselves fat, take drugs, etc. It's like you might treat something you borrowed from a friend—such as a book. You'd take care of it, not scribble in it or let it get wet. That's why taking your own life is so wrong. It would be like destroying something that wasn't yours.