It may be helpful to rearrange the words and shorten as: The fresh Spring never comes completed to hill, etc. without her cuckoo-strain.
There's a sort of joke, wherein the woman complains that her husband never says, 'I love you' anymore, to which he replies, "Look honey: I said I love you on our wedding day. If that ever changes, I'll let you know."
I think the writer is expressing her need to hear the words in order to feel loved. He may get a bit tired of saying them, but she never tires of hearing them.
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