What is the lesson in the poem "The Tame Bird was in a Cage" by Rabindranath Tagore?

One of Tagore's lasting legacies to all of literature is
that he composed works where the "lesson" was not something entirely easy to grasp. 
There is little simple in his work.  The poem is question is a great example of this. 
If there can be a clear lesson pulled from it, there might be a statement being made
about the nature of the balance between dependence and independence.  The poem discusses
the fundamental challenge in any relationship where individuals have to step outside of
their own condition and absorb the reality of another.  In a setting where relationships
are predicated upon "equality" and "partnerships" of proportional share, this is quite
challenging. In the end, one of the birds will have to sacrifice their own condition in
the hopes of enjoying the shared love with the other.  Either the caged bird must risk
the outside world or the free bird must capitulate to living in the cage.  Tagore writes
for a world audience, and his ideas are ones that challenge both the "West"  and the
"East."  This would be one of those conditions that Tagore brings out where both
cultures would have challenges in trying to grasp his overall meaning.  In the end, the
lesson that emerges is that both birds, clinging to their own notion of the good, are
left to ponder only what could have been in a world of what is, where loneliness and
forlornness is the only condition.  In order to fully grasp what can be when two people
are in love, Tagore seems to be suggesting that some level of discomfort and risk to
sacrifice one's condition in the hopes of another is the only way to find happiness with
another.