The second doesn't derive from the first.
If anything, "life is short, so don't obsess about its duration" is a better guideline.
>I always hear stuff like "whatever man, you only live once." It seems so irrational to use that to justify bad habits. If you only live once, should you not try to increase the length and quality of that life by trying to be in the best health possible?
"Bad habits" are some of the most quality time people get to have.