What does one life matter? Hmm, I suppose it depends on the life.
If that life is Hitler, his life mattered (in a negative way) to millions of other people and their families.
If that life is Mother Theresa, or the Pope, their lives comfort millions of people and their families.
If a doctors life, his/her life helps heals other lives.
Drunk driver/drug pusher/addict? His/her life cause pain to other lives.
Think about our ancestors, each of them had to survive thru whatever hardships and struggles they had for us to be here today. The Bubonic Plague, or any other illness, wars, famine, flood, drought, barbarians, pregnancy complications and other medical issues that are routine today, ruthless rulers, and so much more.
Really, think about how low the odds have been stacked against each of us to be here today instead of never even existing. From the first humans in our ancestry line, thru thousands and thousands of years of human civilization to the present. Each one of those lives mattered to us, to the person each of them mated with and onwards to our progeny in the future.
So even if one life matters a little, a lot or not at all during its lifetime, it matters a whole lot thru the reverberations caused by that one life in the course of human history. And since humans are a social species, that means everything.