Get out your tissues, Shmoopers, this one's a tearjerker. The speaker of "Lycidas" has just lost his best bud. Needless to say, our guy is sad. He repeatedly reminds us that not only are he and his fellow shepherds missing a friend, but nature, too, is missing a buddy. Even the plants and trees are mourning Lycidas because they no longer have someone to entertain them. It sounds like this guy really made an impression. Even by the end of the poem, when the speaker says Lycidas has been reborn, the sadness seems to persist.
Although nobody likes to be sad, the speaker implies that there is something therapeutic about mourning. Writing the poem is an act of sadness and solace, so in a weird way, the poem itself, by its very existence, stresses the importance of experiencing sadness and working through it.
The speaker's sadness is questionable because it's conventional, and because at times we feel as though he's more worried about his own death than Lycidas'.