"Slow, Slow, Fresh Fount" is a lament, which is fancy way of saying that it's all about mourning. In this case, Echo, the poem's speaker, is mourning the fact that a boy she loved, Narcissus, is dead and gone. Nothing remains of his memory but a daffodil, which is sad enough to begin with. Add to that the fact that Echo can never have her man, and you have a poem chock full of woe.
Echo is only as sad as nature allows her to be. Her grief is tied to the fount, and the flowers, which have to reflect her sadness back to her in order for her to feel it.
Echo imagines "woe" singing her song, and she also encourages the flowers to "droop," to distance herself from the sadness she feels.