My body belongs to me, I don't know whose yours belongs to. (That was a retorical statement, I know exactly who you think yours belongs to so don't bother saying it). The responsibility for my body belongs to me. The responsibility for everything that it does and says and thinks is mine and mine alone.
If hurting yourself does hurt someone else then the first statement applies. But it is not the harming of yourself that is wrong, it is the consequences that it has. If it has no consequences to someone else it is not wrong, just stupid.
I have to say that I disagree with that. Our bodies *are* our own responsibility, but I wouldn't say they were ours to do what we like with.
Firstly I'd say that there is no act by which if we hurt ourselves it will not have consequences for others. A person's actions reflect what his heart (inner feelings, attitudes, etc) are like.
Jesus said: "But the things that come out of the mouth come from the heart, and these make a man 'unclean.'" (Matthew chp 15)
By this he meant that all the hurtful things that come out of our mouths of course originate in a man's heart / soul. So if we e.g. hurt ourselves by filling our minds with hate, or porn, or whatever else, it will naturally spill over into the way we act with others.
Ted Bundy, a notorious serial rapist and murderer in the US who was executed some years ago, started down the path he was on by an interest in porn which ended up consuming him. I think they even made a slasher movie about him... I don't whether it included the bit at the end when he became a Christian in prison though! (Though this of course does not excuse the fact that he had to pay the penalty for what he had done, but does show that no-one is so bad that they can't have their sins forgiven by Jesus).
Though this is an extreme example, no-one can hope to trifle with sin without being burnt somehow: it's a snare, and sin can only corrupt and destroy.
Secondly, by hurting ourselves, then we hurt what God has given us. Our bodies were given to us by God to do good with.
And for Christians:
19"Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your body."
However, since the Adam & Eve and the fall, we have been moving in the opposite direction. Even though only God can offer us true, lasting satisfaction, we continue to hurt ourselves by gorging on wordly pleasures: drink, and drugs, and promiscuity, which can never give us lasting satisfaction, instead of striving to help others and do good, which we are incapable of doing without the help and forgiveness of Jesus.
(Sure, e.g. intoxication make make a person happy for one night, but you have to keep on amd on getting drunk in order to retain that happy feeling: getting drunk that one night has not brought you lasting happiness, and alcoholism will hurt ourselves, and the body that is not our own but only on loan: the body we were meant to do good with).
By continuing on this path of hurt, we ruin our bodies and ourselves, selfishly wasting our bodies on earthly pleasures that don't last. Given that it is God who gave us our bodies, us misusing them to hurt ourselves is sin, and stupid to boot.
For example, say somebody, as a charity worker, was given a very generous donation for charity, but decided to spend it on themselves... it is the same with our bodies.
But then this second point depends upon whether God really does exist, and whether the Bible, the miraculous acts of God and the life of Jesus, and the [overwhelming] historical, scientific, and self supporting evidence for the Bible are true.
Nonetheless, the above could be discussed at length.
But despite man's best efforts to disprove God, from the proof I have seen, I will say to anyone and everyone that the Bible is true, and Jesus able to forgive us. Lest anyone think I'm being judgemental for what I've said above, it is only becuase I know that I am guilty of all of it that I am so grateful that Jesus has forgiven me.
Form the bottom of my heart,
Best Wishes to you.