Why do I grieve when I choose to "feel" a joyous moment?

You may suffer from depression. Crying from joy is not unusual, but feeling sadness upon feeling joy is unique. I know exactly what you mean though.

Do you have trouble sleeping? Irritability? You don't need to be sunk deep in the blues to have a brain chemical imbalance.

This may seem like a strange question, but do these episodes bother you? Do you in a strange way enjoy them?

Tot it might be a matter of chemistry, of which not much is known as supposed.
Presence or absence of small amounts can have profound effect

https://www.google.ca/webhp?hl=en&tab=ii&ei=bHtJU6aEOeXwyQHlyoAY&ved=0CBEQ1S4#hl=en&q=happiness+chem+in+bloodstream&spell=1

https://www.google.ca/webhp?hl=en&tab=ii&ei=bHtJU6aEOeXwyQHlyoAY&ved=0CBEQ1S4#hl=en&q=happiness+hormone

And of course there's melatonin

https://www.google.ca/webhp?hl=en&tab=ii&ei=bHtJU6aEOeXwyQHlyoAY&ved=0CBEQ1S4#hl=en&q=5-htp+promotes+hormone

Everyday substances can drag you down. Every so often in late morn or early afternoon I'd feel just wonderful for no apparent reason , that is until I discovered later in the day that Id forgotten to take my hypertension, cholesterol, and kidney meds

It is not unusual for people who feel overwhelmed with happiness to cry. It's called "crying tears of joy."

Today I was at a nursery and saw a huge Queen Madam begonia that so touched me I wanted to cry. I've been thinking of it all day.

There is nothing wrong with you. Count your blessings. Life is good!

I seldom mesh smoothly with Finn, but I agree with the possibility of depression. It's not as unusual as you might think. Many people deal with minor depression that will surface as you described. I realize many folks still stigmatize depression, but ask your doctor for an opinion or a referral. The right medication can do wonders. The medication doesn't make you happy all the time, but it will correct the imbalance so you will experience normal emotion. I'm certainly not a doctor, but I can always tell when family members go off their meds. Depression is much easier to treat than bi-polar disorder. I don't want to imply my family gets crazed when they don't use the meds, they just tend to become irritable. We all know its nuts not to treat diabetes, well get treatment for the depression or at least talk to the doctor. Men seem to find it much more difficult to talk to a doctor about 'feelings', please don't let that stop you. A thorough exam with blood work will rule out thyroid or rule it in, don't lose sight of the possibility that it could be something other than depression. Just get checked out. Good Luck

Thank you all for your heart-felt responses which I value greatly. I am going to investigate the "depression" theory. I have read that some people who have depression are not even aware that they have it.
So far, there seems to be a tendency in me to suppress the sadness and grief when an "episode" occurs. I am going to try to open up and let it out and see if I can cleanse whatever is causing this sadness. At the very least, I may be able to find the source and that's half the solution.
I will also mention this to my doctor and see what she recommends.
Thank you all and if I have news, I will let you all know. You have given me hope.