He says to avoid poisons like Wolf's-bane, nightshade, and yew berries. Likewise, if your mistress is angry with you, look into her eyes and feast on their ephemeral beauty.
Contrast is the key to pleasure.
Melancholy is not the moment for death, but an opportunity for a fine experience. It is the fine balance between pain and pleasure that is ideal. The final stanza rephrases this idea. Commentary Keats does not tell his readers to avoid pain or to try and cheer themselves up. He urges them to balance their pain with tremendous pleasure and relish the contrast.