The tone in Shakespeare's Sonnet 14 is one of loving reverence. The speaker tells us that he has studied astronomy and can understand the stars in their physical sense, but he cannot read them as an astrologer would. He cannot 

...tell of good or evil luck,
Of plagues, of dearths, or seasons' quality;
Nor can I fortune to brief minutes tell,
Pointing to each his thunder, rain and wind.

After admitting this to his subject, the speaker says that he can read the stars in her eyes:

But from thine eyes my knowledge I derive,
And, constant stars, in them I read such art
As truth and beauty shall together thrive.

Here is where he illustrates his reverence. Having studied the stars, he reveres her, because he sees "truth and beauty" in her eyes and not where he really should see it - in the stars.

In the last three lines the speaker adds a sense of love to his words. He says:

If from thyself to store thou wouldst convert;
Or else of thee this I prognosticate:
Thy end is truth's and beauty's doom and date.

He's saying that her "end" would be the end of truth and beauty. If she does not live on, through children, truth and beauty will die with her. These are the very loving words that add to the reverence of the opening lines.