Another question: Why do kids climb trees in the first place?

Answer: Just for the fun of it!

As an ex tree-climber (and I still do sometimes, when nobody is looking....), for me the main point Frost is trying to get across is the importance of pursuing one's own aspirations, no matter how insignificant and futile they may seem to others. For life to have meaning, a person must set his own goals then do his best to achieve them and not get "lost" or sidetracked along the way.

"Birches" is a very complex poem subject to many interpretations. It is characterized by antitheses, including imagination/reality and youth/adulthood. In surveying birch trees that have been bent to the ground, the narrator likes to imagine that they were ridden down by a boy at play. He knows, however, that in reality they were brought to the ground by an ice storm. Having acknowledged this truth, he returns to his imaginary boy and takes delight in the boy's joy and expertise has he learns to "[kick] his way down through the air to the ground."

This joy of youth is soon contrasted with the realities of adult life as Frost writes these lines:

So was I once myself a swinger of birches.
And so I dream of going back to be.
It’s when I’m weary of considerations,
And life is too much like a pathless wood
Where your face burns and tickles with the cobwebs
Broken across it, and one eye is weeping
From a twig’s having lashed across it open.
I’d like to get away from earth awhile
And then come back to it and begin over.

"Birches," then, becomes metaphorical for these antithetical stages of life: the joy and freedom of youth contrasted with the struggles, burdens, and "wounds" of adulthood. In Frost's poem, youth is much to be preferred: "One could do worse than be a swinger of birches."