In Book III, Lucretius notes that “fear of Hell which blasts the life of man from its very foundations, sullying everything with the blackness of death and leaving no pleasure pure and unalloyed.” (This will appear differently depending on your translation.) Lucretius believed there was nothing after death, so to fear it is silly because that is to fear nothing. He felt that the fear of death and man's creation of the wrath of God inhibited pleasure and happiness as much as anything. Therefore, living for the present is the best way to find happiness.We should live morally and to be happy, but should not lose any sleep over eternal retribution.
Happiness is obtained by accepting the way the world is. And this comes with having an understanding of the nature of things. Anything dealing with death, the afterlife and other unknowable things not affecting mortal life are irrelevant.
Pleasure is sometimes defined by Lucretius as the absence of pain. Pleasure can be temporary, regarding physical or emotional sensations but pleasure can also be sustained by having a content mental attitude, one that is not preoccupied by fear (namely, of death). Mentally speaking, lack of psychological anxiety is one key to happiness.
Pleasure and happiness, in terms of love, can manifest in the good and the bad. One can be loved and loving, but being love-sick and/or experiencing unrequited love can be devastating. In fact, even two lovers having mutual love for one another can experience this dichotomy of love. In Book IV, he writes,
Lust, gathered in the thews, hath spent itself,
There come a brief pause in the raging heat—
But then a madness just the same returns
And that old fury visits them again,
When once again they seek and crave to reach
They know not what, all powerless to find
The artifice to subjugate the bane.
In such uncertain state they waste away
With unseen wound. (1590-99).
(This might also differ, according to your translation.)
Using Lucretius' philosophy to essentially live “for the now” and not be afraid of death or loss, the lover would be better off if he/she accepts that there is an ebb and flow to this kind of passion/sex and it is best to enjoy it while it lasts and not agonize when it is not present.