When to use "lives" as a plural of life?

I am confused when talking about a general idea using "our life" when sometimes I feel like using "our lives". Please tell me the correct answer with appropriate explanation.

These examples illustrate when you would want to use the singular versus plural of life:

1. Our lives have been very different.

2. Our life together has been very happy.

In (2), I imply that we have shared a life, hence we jointly have had one life. In (1), I imply the opposite (different lives have to be counted separately). Consequently, the singular is felicitous in (2), but not in (1).

Felicitous does not mean obligatory, though. You can also say:

1. Our lives together have been very happy.

This is possible because we each have a life and it is possible to spend them together. Personally, I prefer the singular though.

By contrast, you completely change the meaning by using the plural in:

1. Our life has been very different.

This no longer means “different from each other’s lives”, but implicitly contrasts with someone else’s life (or lives).