There is a difference between the nouns "life" and "lives."
Life can be defined as a span of time from birth to death.
1. A LIFE is used as a Singular, Countable Noun
For example:
You are standing next to the grave of a man. He lived to be 100 years old, traveled the world and was very happy. You can say:
He had a good life.
2. THE LIFE is a Specific, Countable Noun. Use "the" in front of the countable noun "life" when you want to get specific.
For example:
The life she lives is courageous. Her life is courageous.
3. LIFE can also be used when we want to express LIFE as something that is a General, Non-Countable Noun. We use this when we refer to the experience of living life. This situation is similar to when we use other general, non-count nouns like air or water.
For example:
Life is beautiful!
Life is full of surprises!
4. LIVES is the plural noun for life.
a. You can use it as a Specific, Countable Plural Noun.
For example:
There were five lives lost in the plane crash.
The lives of future generations depend on us.
b. You can also use this Plural Noun in a more General way. This refers to the collective number of individual lives.
(my life +your life+her life= our lives)
For example:
We want to know how to live healthier lives.
Our lives have impact.
Remember to live (v) is a verb.
You only have one life (#1) to live.
I wish I had the life (#2) you live.
Living life (# 3) positively will benefit all lives (# 4b).
If you believe in reincarnation, you have lived many lives (# 4a).
Let's look a little deeper.
Notice the difference in meaning:
1. There is evidence of life (#3) on Mars.
There is evidence that life exists. This is life in general. There may be water on Mars. There may be oxygen on Mars. There may be bacteria. This is evidence of life.
2. There is evidence of lives (#4) on Mars.
This means that there are individuals living life on Mars. This is very different than there is evidence of "life." This also sounds WEIRD and would NOT be possible to say because we don't know any Martians (the green aliens who live on Mars) who live there (yet!). Even if we did know Martians, we would not say it this way.
A better way to express this would be:
Martian lives (#4) are cool. Martian lives (#4) are better lived than human lives are.
Evidence shows that Martians live happy lives (#4). Life (#3) in the solar system is wondrous. Alien life (#3) may exist. Perhaps one day our lives (#4) will be impacted by life (#3) from other solar systems!