In the first quatrain of the poem, the speaker sounds really depressed. His metaphor of the tree that has lost all its leaves makes it sound like he feels pretty hopeless about where he is in life. The poor guy isn't completely old yet, but with his youth definitely gone—far gone. And the next quatrain sounds pretty much the same downer note; the sunset imagery is pretty, but the final imagery of Death "seal[ing] up all in rest" doesn't leave much room for optimism.
The same could generally be said for quatrain 3, but here at least the speaker sounds a bit more determined, if not exactly hopeful. Even though the fire the speaker compares himself to is nearly burned out, it is still burning, and giving off a little bit of life and warmth.
Finally, the concluding couplet sounds kind of determined too. The speaker doesn't ask the listener to stand by him; he simply says, confidently, that the listener will. No doubt. So even though the speaker never imagines that he will live on in the afterlife, he still does have the sense that there will be the consolation of companionship and love in this life. That is, until he finally croaks.