Two “individuals” who might be said to pursue individual well-being in Robert Frost’s poem “Come In” are the bird and the man who listens to the bird.  The bird might be said to pursue individual well-being in spite of internal and external demands in such ways as the following:

• The bird continues to sing despite the darkness that surrounds it.  Rather than giving in to the “external demand” to cease singing when night arrives, the bird chooses to continue singing.
• The bird sings despite (or perhaps because of) another external challenge: apparently it has a less than ideal place to perch.
• The bird sings despite (or perhaps because of) an internal demand: the need for sleep.
• Since it is too dark in the woods for the bird to fly and discover a better perch, the bird makes the best of its present situation by singing. Perhaps it sings out of some discomfort and annoyance with its present situation, but at least it sings, thus making the best out of a less than pleasant circumstance:

Too dark in the woods for a bird
By sleight of wing
To better its perch for the night,
Though it still could sing.

Meanwhile, the speaker of the poem might also be said to pursue individual well-being in spite of internal and external demands in such ways as the following:

• Although he imagines that the bird is calling him to join the bird in the dark forest, he refuses to do so.  Perhaps the dark woods are associated symbolically with a kind of mental darkness, and perhaps the song of the bird is also associated symbolically with a kind of lamentation, as the following lines suggest:

Far in the pillared dark
Thrush music went --
Almost like a call to come in
To the dark and lament.

Despite his attraction to the song of the bird, however, the speaker resists the temptation to enter the woods:

But no, I was out for stars;
I would not come in.

Do these lines mean that he symbolically chooses light over darkness, life over death, the pursuit of happiness over a giving-in to sadness? In any case, the speaker chooses his own well-being in spite of being attracted (perhaps even tempted) by the dark woods and the sound of lamentation.