It depends on the context. If you are talking about a general situation in which what you are saying is applicable to everyone and not just a single person or group, then you should say "lives."
However, if you are talking about a single person, or a group of people whose lives are directly related to one another, then you'll use the singular "life." For example:
"Our life together as a married couple has been very beautiful." You say "life" here, because the couple is consider that their individual lives have come together and are assumed to be one unit in this case.
"Our lives were forever changed by the incident" In this case, an outside occurrence affects many people at once and therefore "lives" is used in this context.
I'm not sure how else to explain it, but I hope these examples and my explanation helps. If you have further questions, please let me know.