When you love someone, is the joy (the sense of completeness it can offer a person) directly proportional to the amount of anguish one experiences when that love is withdrawn?

No. It’s an interesting theory, but it doesn’t make sense. Why would you think that there is any sort of symmetrical relationship between love given and love withdrawn? What’s your theory behind that?

Your theory assumes that relationships are zero-sum and static. It doesn’t take into account that people change over time, and the depth of their love changes as well.

Suppose you invest a lot of live in someone and he/she dies suddenly? No symmetrical relationship there.

In short, @luigirovatti – I think your assumption is wrong.

I don’t think so.
I am not saying that it wouldn’t be painful but if you place all your value on whether or not someone loves you, you will be crushed like a bug if they leave you.

That’s a broad stroke of a point of view for a generalized answer. To each his/her own, would be more logical and realistic.

I would imagine so.

From firsthand experience: I never realized I loved someone (who loved me immensely) until she was gone (e.g. died). I suffered anquish for a couple of years because I never told her I loved her (which she wanted) and never even realized it.

No.
Better to have loved and lost than NEVER to have loved in the first place.

Wiktionary. better to have loved and lost than never to have loved at all(Proverb)
Having experience of love, even if it ended, is better than having no experience of love. better to have loved and lost than never to have loved at all(Proverb)

Love is worthwhile despite the pain involved in separation.

How do I love thee?
Let me count the ways.
How do you not love me back?
Let me count the ways.

( Yup. It can be quantified! )

Not sure how that addresses the question of the anguish that is felt upon a breakup, which was the question @Inspired_2write.

I think that if you were truly, honestly, deeply in love with someone, the anguish at them leaving would be far greater than the “anguish” you would feel over someone you loved much less leaving.

No, it depends on the relationship.

@Dutchess_III
That is subjective and depends on the person as to how and how long they anguish.

By the way YES I have experienced that anguish for three years until I began to understand from whence that pain comes from originally.

For me it was from NOT feeling worthy enough to be loved probably compounded from childhood experiences of being left out and not truly loved or valued.

I now appreciate that I had that Requited Love abet only two years but nevertheless happy to have received it in my lifetime.
It cannot be measured like a ruler or weight on scales..to do so would render it useless as feelings cannot be measured.( subjective)