Central meaning: the beauty of this young lady will not fade because Shakespeare wrote a poem about her. Mortal beings can and will achieve immortality through the everlasting power of poetry. Note his high opinion of his own poetry!
The whole poem is in fact a (negative) answer to the question in the first line. And the reasons?
First of all: she’s more beautiful (difference of degree).
But more essential (v.3-8): he’s talking about summer, but between the lines we can read the contrary (his love is more temperate, not so “hot”).
