It should also be mentioned that the relationship is based on the “melody” that the poet explicitly refers to in the second and third stanzas. The expression “Heard melodies are sweet, / But those unheard are Sweeter” refers to the melody played by the “happy melodist” of the third stanza. The melodist plays his “soft pipes” and this creates an environment for perpetual happiness. The happiness that the “bold lovers” and “happy boughs” enjoy is eternal too. Keats captures an eternal moment of happiness and compares it with "unheard melodies". As the poet will enjoy the unheard melodies perpetually, the “bold lovers” and “happy bough” too would enjoy their happiness in perpetuity.
When the speaker speaks of the happiness of the lovers who never will grow old, and who are always just at the point of highest expectation of that first sweet kiss, and of spring which will always remain warm with leaves on the trees and birds singing in them, he is also leading up to imagination. He imagines how all these things feel as he is comparing how he has felt in similar situations. It is also true, then, that the "heard melodies are sweet..." these are the ones which are played that we can all hear and pass judgment on--do we like it or not? But "those unheard are sweeter" alludes to the ones we imagine the lute player is playing...if we imagine it, it will always be a beautiful and sweet tune. There is nothing to judge, and it will be sweet to EVERY listener because every listener imagines that which is beautiful to him or her. Therefore, we are all guaranteed a lovely and sweet melody...how can we not be happy then?