Both Browning's and Neruda's sonnets present love as a feeling that should not cling to anything temporary or transient. They try to tell us what love is by telling us what it shouldn't be. Browning introduces the following negatives: smile, look, gentle manners, the need for comfort. She points out that these things may pass. Even though they are conventionally understood as signs of love, she wants something better and more stable than that. She wants to be loved for love's sake, as love is eternal.
For Neruda, on the other hand, love is something he can't describe by likening it to particular, specific, well-known things or feelings. It is indescribable and unknowable, and therefore indefinite. It escapes any kind of attempt to fixate it by connections to this world.
