Browning's and Neruda's sonnets present love as a feeling or sensation that should not grip to anything impermanent or passing. They try to say what love is by expressing to us what it shouldn't be.
Browning presents the following negatives like smile, look, gentle manners, and the need for relief. She emphasizes that these things may pass. Even if they are usually understood as emblems of love, she wants somewhat better and steadier than that. She wants to be loved for love's sake, as love is everlasting.
On the other hand, for Neruda, love is somewhat he can't define by associating it to exact, precise, well-known things or feelings. It is inexpressible and incomprehensible, and as a result it is indefinite. It escapes any kind of effort to grip it by influences to this world.