My mom's best advice to me has always been not to let hard times in my life make me a bitter person. Today's quote has been misattributed to Kurt Vonnegut, but I managed to find the original blog post for August 16, 2007 by Iain S. Thomas, entitled "I Wrote This for You: The Fur," and believe it or not, the photo he used was also that of a kitten.
"Be soft. Do not let the world make you hard. Do not let pain make you hate. Do not let the bitterness steal your sweetness. Take pride that even though the rest of the world may disagree, you still believe it to be a beautiful place."
Merriam-Webster says bitterness is anger or resentment at being treated unfairly. It's true that when people are bitter, they are generally angry at the unfairness of life. The problem is, who told them that life was supposed to be fair? That whole sense of entitlement is what makes people so miserable. It's viewing the world according to their unrealistic expectations, instead of the way it actually is. Besides, what does "fair" mean, really? How many times have you had people agree that something was fair even when they got the short end of the stick? Conversely, it's very rare for a person who is getting the better part of the bargain to charge that something is unfair.
Another part of the definition says is is something distasteful or distressing to the mind. That's a key point: in the mind, because all our negative emotions originate in the mind. A lot of times, we label something as negative, sad, wrong, or bad and then get upset about it. If we didn't label it, would we get upset about it? Truly, we make ourselves miserable.
The last line of the quote recalls the ending of the "Desiderata," by Max Ehrmann in 1927. "With all its sham, drudgery, and broken dreams, it is still a beautiful world." The physical world is the lowest-density place in God's entire creation, and because this is a learning place, duality is a given here, because how would we know what darkness is if we saw only light? How would we appreciate feeling full if we had never experienced hunger? How could we understand pain if we didn't also experience pleasure? So yes, there are negative things here in the physical world. But they are balanced by the positive, and there is still incredible beauty and positivity in the world, in spite of the fact that a lot of people seem to think it's cool to be jaded and cynical. :-)
(By the way, "Desiderata" is another piece of writing that has been misattributed, because it was included in a book of devotions for St. Paul's Church in Baltimore, Maryland a few years after Ehrmann's death. The compilation included the church's foundation date: Old St. Paul's Church, Baltimore, A.D. 1692. People just don't read very carefully.