The line is spoken by Bottom in William Shakespeare's "A Midsummer Night's Dream" Act III Scene 1. The full quotation is:
[Bottom:] Methinks, mistress, you should have little reason for that. And yet, to say the truth, reason and love keep little company together nowadays.
This passage contrasts reason with the fantastic world of the dream itself, with its magic and fairies. In this scene, Bottom has been transformed into a donkey and Puck has placed the magical drops in Titania's eyes that make her fall in love with him. Both of these transformations lie outside the possibilities of the real world of reason.
Bottom's analysis, that Titania's declaration by love is not inspired by reason, is a statement regarding a particular instance of love, but then he follows it up with a more general conclusion that love is not rational. Thematically, that notion drives much of the play's plot, in that the love of Oberon and Titania for the changeling, the loves of the four young human lovers, and even Theseus' love for Hippolyta,lead them to irrational behavior.
Thematically, however, another major point of the play is that poetry, like love, is not in its nature fully rational, and just as the lovers act on emotions rather than reason, the duty of poetry is to be entertaining, as Puck suggests, and like a vision or a dream, not to contain philosophical or scientific arguments.