word usage - Which one sounds right: "life is short" or "lives are short"?

According to the Oxford Learner's Dictionary, "life" can be a countable and uncountable noun.

The floods caused a massive loss of life
Hundreds of lives were threatened

But if we wanted to say "life" in general, which one sounds right or are they both grammatically correct?

1. life is short
   or


2. lives are short.

Answer

To talk about life in general, life is considered a mass noun. That is to say, to talk about life as a concept, we say life in general; In general, life is short. This very usage is what makes life here a mass noun. Compare Death is inevitable.

However, we can also talk about lives in general. This talks about multiple instances of the singlular count noun life. For instance, we might say, In war, lives are cheap. Here the stress is on individual lives. But you could also In war, life is cheap, with life as a concept.

You could also say, in general, deaths are inevitable, in the medical profession

For 'types' of mass nouns, see this resource by the University of Washington.