In Sonnet 29, the poet describes his depression and low self esteem. He wants to be like other people, like "one more rich in hope", and with different features, friends, and station in life. But in this dark moment, all he has to do is to "think on thee, and then my state/Like to the lark at break of day arising/From sullen earth, sings hymns at heaven's gate", and he is suffused with joy. The love of this person is enough to lift him from the deepest depression. Sonnet 116 talks more about the unchangeable nature of love, and that it is not based on appearances or mutable things. Love is an "ever-fixed mark/That looks on tempests and is never shaken." Something this permanent and unchangeable must be important, because it is one of the few things in life which lasts. Sonnet 130 is more wry, but it is no less unstinting in its praise of love. He describes his mistress as unlike the poetic ideal as can be imagined -- instead of golden threads her hair is "black wires", and her eyes are "nothing like the sun", but still he holds that "by heav'n I think my love as rare/As any she belied with false compare". Though his mistress is not as beautiful as the imaginary women described in poems his love is still as rare and valuable as the unreal loves in the poems. This is the most realistic, and perhaps most moving, description of the importance of love of the three poems.
There are several ways the importance of love is shown. In sonnet 29, it is shown through contrast: the speaker can be very depressed, but if he thinks of love, he's revived.
In sonnet 116, the power of love is shown through the extremity of the claims made for it. Shakespeare makes absolute claims: " Love is not love/ Which alters when it alteration finds…" That is to say, love never changes, no matter what happens. You can use it to guide your life.
Both strategies are combined in sonnet 130: the speaker contrasts his love's beauty with other things (finding them superior), makes extreme claims for his love and the woman, and uses that emotion to lift his heart.