"The heart knows its own bitterness, and a stranger does not share its joy."
The word bitterness is the Hebrew word which is myrrh – the fragrance which oozes brokenness and pain. It was the perfume that was brought by the Wise Men who came from afar to present gifts to the King of the East at Christ's birth. It was also one of the spices that was brought to anoint the body of Jesus when He died. It expresses the result of tragic, unpleasant, and bitter experiences in life.
The word translated joy is the word sameah which means joyful, merry, happy – all the occasions that bring lightness of heart and joy.
This proverb is trying to tell each person that there will be things that will happen to you that no one can fully share with you because they are not you – even those who went through the event with you. There will be tragedies and blessings that emotionally will overwhelm you and others will just not be able to fully comprehend or appreciate or enter into those emotions with you. They will be yours.
I believe that this suggests that when one is overcome, one should not be quick to condemn others for not understanding what we've been through. This is true of teenagers getting angry with parents who don't understand how awful it was when so and so broke up with you or how mortified you were when the embarrassing thing happened. Or the flights of rapture and wonder when so and so looked at you or talked with you. This is true when the boss does not understand what the passing of your grandparents means to you and why you need to have a few days off. This is true when you face a bankruptcy or a relative who was murdered. These may have no connection to the person listening; and even if they do have a similar experience, they will not be able to really relate to the level of your emotion. So cut them some slack if they don't get it right.
This is very similar sentiment to the proverb which says that if you are wise, you are wise for yourself; if you are a fool, you alone will bear it. What immediately pops to my mind is that many people do things to impress others or connect with them. But realize that what you actually do, you will actually have to account for so don't do things that you shouldn't just to impress or connect with another – especially if it is immoral.
The thrust of the proverb is that there will be depths of emotions that only you will know about and be able to experience; and no matter how much you write about them or talk about them, they are yours. It is good to talk about them and good to write about them and this does, at times, act as a catharsis but never forget that they are your emotions.
I can remember when a good friend of mine passed away after being burned alive in a tragic propane fire. I just had to be alone and cry for a long time. There were emotions in me that I had never experienced before.
The wise person realizes that the depth of bitterness and injustice one feels by a particular event in their life may not be able to be shared by others, so don't jump on others who just don't get how much of a life-changing event this was for you.
I see this many times when I watch people not listen well to their friends at church or work. The one person is telling an emotionally-laden experience, and the other person checks their watch or is obviously bored or stops listening. And the speaker is highly offended. This often marks a turning point in a relationship. "You didn't listen to me when I really needed you to." This type of assessment is usually overly harsh and does not take into account the wisdom of this proverb. The other person just cannot enter into the depth of your emotions – whether good or bad.
Now I do have to say that we all can get better at listening skills, and in that way, show a higher level of love and care for those whom God sends our way.