Life is short relative to what? You will only experience one life, so to you, life is both infinitely long and only a brief moment. Life is what you make it; so you should make the most of it by applying scientific rigor to maximizing what is important to you.
Sentiment analysis as a rigorous process exists to help machines that do not possess natural emotion processing to understand human sentiments. If you are the one having your own sentiments, why would you offload that to a machine? Unless you have a broken emotional processing center yourself, but in that case I doubt you could record meaningful sentiments either.
Sarcasm, for example, works very poorly with SA, and if you tend to write sarcastic prose, SA will be a waste of time at best, and counter productive at worst.
I would say that's still a subjective measure; it sounds like you disagree?
“I love these chips” is subjective. Rate-of-saying “I love (.)!” is objective.
The absolute rate may or may not be important (I’d guess low importance), but changes* to the rate can indicate that a life changing event had a much larger impact than one may be subjectively conscious of, especially if the change was gradual — a loved one becoming gradually more ill or more emotionally distant, for example, slowly enough that one does not realise that light and joy has begun to leave one’s life until after the recycling bin overflows with beer bottles.
An objective measure, like weight or hours of sleep or time to run 5km doesn't have this property.