Well the best answer would be, don't worry since there is no god there's nothing to be upset at.
There are a few other reasons why I replied though.
Firstly its wonderful that you are alive (still I hope). Out of the millions and millions of babies that could have been born, you were. This may be because you were the strongest, or there may even be some luck (ideal circumstances for you) involved. Whatever it was, YOU were the one who survived.
It is true that we require other lifeforms to stay alive. Because even chemically made nutrients and other scientifically generated foods at least have plant life or microscopic cell life in them. But then again so do we.
As disgusting as it may sound, we are literally full of living cells organisms that are not us, but without them we would most certainly die. These 'living' lifeforms do die extremely often and new ones are also born. All this inside of us, in a never ending balanced body we call us.
This is the life form you are. In some sense you are extremely privileged to be allowed this life, where MOST weren't. Let alone still borns or even child deaths. You should be thankful for your own life. You are able to see and be a part of the wonders of the world (and universe) in which you live, plus grow up and experience everything the world has to offer including love (not just sex!) and possibly having your own family one day. The joy felt with having your own children (or loved ones) ideally is not to be missed either, most would even give their entire life for this alone.
Pain and suffering is a part of life too. Think of this way, if it was all good all the way long, not only would it be boring but you'd never fully appreciate health and well being. Pain is just a way of our brain trying to tell us to act on something, ie Don't touch fire or it might hurt (and eventually kill you!) Or understand that all life is precious take care of it.
Age has its own benefits too. We can see things even more clearly, help others when we can, and build stronger connections with others and our surroundings. Many 'old' people don't want to be children again, but would love to have their own life over again. Because life is meaningful, and if you like it or not what you do affects others, even me writing in reply to you. It provides memories and experiences good and bad and everything in between. Its true to say that the majority of old age people pass away content that they lived a life worth living, and were happy and thankful for it. Knowing it was all worth it.
Its not sadistically cruel that we live. Its the best time ever, enjoy it whilst it lasts.
By the way, many people of our world commit suicide (or attempt it) when they feel there is just no hope any longer. They should seek professional help before doing anything critical (usually just a phone call to lifeline to begin with). Because I can tell you now that every life is worth living and fighting for, just wait and see. :)