In just four lines, the speaker describes a moment of sudden, quiet understanding. Grief, the poem says, filled their eyes with "leased doors." It's a strange image, but a useful one: a leased door isn't yours. You can walk through it, use it, even feel at home behind it, but it belongs to someone else and the arrangement will end.
From that picture comes the last two lines, where the speaker sees that "all I own / I am not to keep." The realization is plain and unsentimental. Everything we hold, whether houses, objects, or perhaps even people, is on loan.
The title, "The pain of others," suggests the speaker is mourning someone else's loss, or perhaps looking at other people's grief. That outside suffering seems to turn the speaker's attention back to their own life. The mood is hushed and sobering, less a cry of sorrow than a calm, slightly chilling clarity about impermanence.