What does ‘On its good days’ of “On its good days, all this (live for love, die for love, kill for love) seems to make perfect sense mean?

There is the following sentence in the Time magazine’s article “The science of romance: Why we love,” (Jan. 28, 2008), dealing with the mechanism of Love: http://www.time.com/time/magazine/article/0,9171,1704672,00.html,

“People compose poetry, novels, sitcoms for love," says Helen Fisher,
  an anthropologist at Rutgers University. “They live for love, die for
  love, and kill for love. It can be stronger than the drive to stay
  alive. 
On its good days (and love has a lot of them), all this seems to make
  perfect sense. Nearly 30 years ago, psychologist Elaine Hatfield of
  the University of Hawaii and sociologist Susan Sprecher now of
  Illinois State University developed a 15-item questionnaire that ranks
  people along what the researchers call the passionate-love scale.”

I cannot get the idea of “on its good days.” ‘Good day’ here doesn’t seem to be a usual greeting word when parting from someone.
I can understand “love has a lot of good days,” but I don’t understand what “(On its) good days” accounts for, and how “good days” are related with “Live for love, die for love and kill for love make perfect sense.”
Can you exlain me?

It's nothing to do with the idiomatic greeting "Good day".
When one (of those people who live/die/kill for love) has a good/positive/enjoyable day, they naturally feel their outlook makes sense/is vindicated.
And that happens often, because such people tend to have lots of "good days".

It's also possible it's the article writer to whom live/die/kill for love seems like a sensible life-plan. The words themselves are ambiguous, but I'm swayed to the first interpretation because of the reference to such people having lots of good days.
Since presumably the writer isn't a "love extremist" herself, she wouldn't have the good days. And thus probably wouldn't think in terms of such an outlook [only] making sense on those days.