How we cite our quotes:
Bitter constraint, and sad occasion dear,
Compels me to disturb your season due (6-7)
Duty calls. The speaker implies that this song, this tribute to his friend, is something of an obligation. It is a "constraint," not something voluntarily undertaken, at least not completely. This might be because the speaker doesn't want to use his poetic gifts for such a sad subject. Or maybe he is so overcome with grief that he can't quite control his own decisions.
He must not float upon his watery bier
Unwept, and welter to the parching wind,
Without the meed of some melodious tear. (12-14)
The speaker wishes that Lycidas could hear this elegy, or "melodious tear." We can't help but notice that he describes the poem with a reference to water ("tear"), which is fitting considering that Lycidas met his death at sea. The fact that the tear is "melodious" suggests that the speaker is converting grief into art, beauty, poetry. We have a feeling Lycidas would be pleased.
What could the muse herself that Orpheus bore,
The muse herself for her enchanting son
Whom universal nature did lament. (58-60)
By comparing Lycidas to Orpheus, the speaker tells us just how much of an impact Lycidas' death will have. Even "universal nature" will be bummed to hear the news. Oh, and by the way, it is a convention of the pastoral elegy to describe all of nature mourning for an especially beloved poet. Milton is checking off all his boxes, like a good poet should.