Lycidas has been tragically drowned at sea. In that sense, nature is responsible for his death. Yet the waves that sent the young swain to his untimely demise are utterly indifferent to human suffering. The same goes for all other features of the natural world, no matter how beautiful they may be.
At first, the speaker appears to find no consolation, no healing power in nature. They are insistent that no amount of beautiful natural imagery can possible provide any consolation for sorrow. Any suggestion to the contrary is nothing but a fond dream or a "false surmise."
Yet as the poem proceeds, the speaker becomes more reconciled to nature, with the richness of its seasonal, cyclical eternity. To be sure, nature hasn't changed; it's still as indifferent to human suffering as ever it was. What has changed, crucially, is the speaker's attitude to nature. They now find some small crumb of comfort in the remarkable capacity that nature has for self-regeneration, as illustrated by the vales casting "their bells and flow'rets of a thousand hues."
These flowers, like so many features of the natural world, will one day die. In that sense, contemplating nature's endless cycle of death and rebirth can help to place the death of Lycidas into perspective, thus providing the speaker with much-needed consolation as part of a healing process. The suggestion is that once we realize where we truly stand in relation to the universe and that the cycle of human death and rebirth parallels that of nature then we will be able to handle grief more effectively.