For an emotionally affecting poem, most of this poem doesn't talk directly about emotion. Yet, through the images that it shows us, emotions really do come through. Then, the end of the poem talks explicitly about joy and sorrow—in terms of life and death.
Line 1: This line, though it seems straightforward as it sets the scene, involves emotion. Think about the emotions that "they" must be feeling.
Lines 2-4: These lines suggest that even though he is long dead, the woman's husband still feels emotion when his wife is buried. These emotions could be joy, or sorrow. It's left up to us; as we read we can't help but tremble a little bit too.
Lines 5-6: These lines are obviously about emotion, specifically, joy and sorrow. They say that joy for life is sorrow for death, but that's not the only situation where this switching occurs. For example, joy in a relationship necessarily means sorrow for the end of that relationship. What other examples can you think of? Giving up chocolate when going on a diet, perhaps? OK, maybe life and death is a lot more serious than giving up chocolate (emphasis on the maybe). But the point is that this poem's discussion of emotion applies to more than just life and death.