the way of the world; the inescapable circumstances of life; just the way things are
A resigned, philosophical expression acknowledging that life is full of unavoidable hardships and that one must accept them. Often used when something unfortunate but predictable happens.
See also: 浮世
It's the way of the world that many things don't go as you wish.
彼が去ってしまったのも浮世の習いだ。
That he left is just the way things are in this world.
From 浮世 (ukiyo, 'the fleeting world') + の (possessive particle) + 習い (narai, 'custom, way'). The phrase reflects a traditional Japanese worldview that life is transient and full of unavoidable sorrows.