This is an excellent question, focusing in real detail on the actual phrasing of the poem.
- The image implied by the verb “Flames” is appropriate to the poem in various ways. For instance, the poem is very much concerned with literal and emotional darkness. Lycidas has died and his body has sunk beneath the waves into what seems a kind of permanent darkness. The flames can therefore be seen as implying the light associated with his resurrection. Just as flames are usually considered the opposite of water, so the fact that Lycidas “Flames” like the sun is appropriate to poem emphasizing watery death.
- The fact that Lycidas “Flames” like the sun also associates the risen Lycidas with God, the source (for Milton and his contemporaries) of all light and life and power.
- The idea that Lycidas now “Flames” like the rising sun is a potent symbol of renewed life. Even the sky is treated here as if it were a living being (since it is said to have a “forehead”). The entire final section of the poem brims with symbols of renewed vitality, and the flaming sun is one of them.