The poet has expressed her concept of ideal love in this poem (sonnet 14). According to her, her lover should love her only for love’s sake. That love should not be conditional in any way, but genuine, sincere and everlasting.

If thou must love me, let it be for nought
Except for love’s sake only.

He should not love her for her smile, look, way of speaking or a similarity and compatibility in thoughts and feelings. She knows that those things are transitory (changes with time). If love is formed based on those qualities, that would not last long.

For these things in themselves, Beloved, may
Be changed, or change for thee, –and love so wrought,
May be unwrought so.

She also forbids him to love her out of sympathy for the same reason. The poet may have nothing to be sad for and may forget to weep in future. This is more so because she would get his love and comfort for long and thus would have no sorrow then. In that case how would the lover show her pity?

A creature might forget to weep who bore
Thy comfort long, and lose thy love thereby.

That is why the poet wants unconditional and eternal love. She wants their love to transcend the passage of time.