First of all it is wrong to treat life as an opportunity. Life is not an opportunity to experience things but the only platform to experience things and therefore should not be considered an opportunity at all. Since existence is by definition everything that exists, since there is nothing outside of existence and no one can be non-existent, existence is not an opportunity to experience things, rather existence is itself to experience things.
The term opportunity implies that someone wanted something and got a chance to realize it. But before anyone was created there was no one who hoped, wanted, aspired, or had it in his interest to have a chance to exist. It is exactly the creation of someone that supposedly created the opportunity, therefore it is incorrect to call the creation of a person an opportunity for someone to experience existence. What other option is there for anyone but existence? What does it mean not to give a chance in this sense? And more importantly, to whom? After all, before the “opportunity” is given, there is no one at all. And if there is no one before the “opportunity”, there is no possibility of denying someone that “opportunity”.
In order for existence to be an opportunity, existence must not only be desirable for those who do not yet exist but there would also have to be a less desirable alternative, and both things do not exist in the context of reproduction. No one has an alternative to existence before creation and no one wanted to exist before creation. Therefore, it is not an opportunity even if the life of the one who was created will be full of positive experiences. This still cannot be considered an opportunity in the principle sense of the word.
The creation of a person could perhaps be considered an opportunity if before s/he was created a person wanted to have positive experiences and it was the creation of that person that opened the door to those experiences. But as just mentioned this is not the case. Reproduction is not to create an option for realization of possibilities for a person who already existed in some way, but literally creating a person, that is, creating someone with needs and desires, the realization of some of which will be accompanied by positive experiences (and failure of their realization by negative experiences). Creating a person is not an opportunity since it does not allow the needy access to fulfilling needs, but literally creates the needy with the needs built in, and in the best case they will succeed in realizing some of them. And even then, this should not be considered an opportunity but rather the creation of needs that are not necessary in themselves, and then the needy must pursue filling them throughout their lifetime. This is not an opportunity, it is an obligation. If the needy does not pursue the fulfillment of the needs, s/he will suffer from lack, and lack of things s/he should not have needed before being created. And now s/he has no choice but to pursue them. Being forced to fill something you didn’t want before you were created just so you wouldn’t experience a lack doesn’t really count as an opportunity.
And as for life being a one-time thing, this is not an advantage but a disadvantage. The fact that life can end in an instant and regardless of the will or conduct of those who live it, or of those close to them, is another reason why life is bad. This does not make them precious but cheap. The reason some claim that the fact people only live once make life precious is related to a fundamental error similar to the one that causes people to look at it as an opportunity, which is treating, even if unintentionally and unconsciously, non-existence as a state someone is in before they exist. Only from such a perspective can someone think of life as an opportunity and as a precious thing, and this is because if someone exists in some way before being created, and if someone wants to exist, then indeed that someone’s creation is an opportunity for her/him and the fact that life is a one-time thing does make the life of those who want to exist precious. But since s/he who does not exist does not want to exist, her/his existence is not an opportunity. And the fact that that existence, which no one asked for, wanted or was in anyone’s interest before it was imposed on someone, can also end at any moment and with very little control over when and how, makes life very cheap indeed.
A distinction must be made here between life in itself and someone’s life for that someone. And there is a connection between the things. The fact that life is so cheap in itself makes it precious to those who live it. The fact that one can very easily die or get hurt, the fact that lives can be destroyed so easily and quickly shows how cheap they are in themselves and precisely because of that they are also precious to those who live them. And this is another good reason not to create a new life since these are cheap in themselves but not to those who are forced to live them. In other words, the fact that someone’s life can easily be destroyed or ended against the will of the one who lived it, does not make it precious in principle, but quite the opposite. Life would be precious if it was difficult to destroy or end it. The fact that it is so easy and so common, is a testament to how cheap it is and by virtue of that also to the terrible cheapening involved in creating more of it.