The passionate shepherd is a pastoral poem with a bit of the carpe diem of the age in it. He says to the young lady, if you like fair weather, simple gifts from nature (beds of flowers, skirts, wool shoes, coral clasps, etc.), then come live with me and be my love. This is a sieze the moment poem where he appeals to the girl's sense of living in the present and going for the temporary pleasures that the scenery, sheep, and his attentions in the spring and summer sun can afford her. He never mentions marriage or long-term love.
The nypmh's reply capitalizes on this. She says, if you could promise that it would last forever...if the summer would be here all year around and your love would never die...then I would be moved. BUT all the things you mention are temporary with no hint of permanent conviction or commitment. Shepherds have "honey tongues" and none of this will last, so why should I come be your love? I can get sweet talk right here where I am, thank you very much. She's not buying what he's trying to sell.