"And yet, to say the truth, reason and love keep little company together nowadays...." How is this passage relevant to the play?
This quote actually sums up the entire play!
Truth, reason, and love do not always go hand in hand. For many people, love is ruled by emotion, not by logic (reason and truth).
Helena is definitely ruled by love. She doesn't have the reason stand up for herself against Demetrius-- she even begs him to treat her like his favorite dog! She doesn't see herself clearly, as a person of value; rather, she only values herself only if Demetrius does. Since he does not love and in fact openly scorns her, Helena feels like an ugly outcast, and she turns against her life long friend, Hermia, with jealousy.
Both Lysander and Demetrius are bewitched when they change their feelings towards the women they adore. In this case, they are not thinking with reason or truth; rather, they are under a spell to love the first thing they see. In this case, they fall for the wrong girls, spouting devotionals that are completely opposite to their original feelings they had for the same women.
Titania, queen of the fairies, is put under the same spell as Lysander and Demetrius, but she falls in love with the mortal simpleton Bottom, whose head, through magic, has been changed into that of an ass. She also pledges her undying love to Bottom, only seeing him through the spell as the love of her life, with reason or truth playing no role in her thoughts.
Shakespeare, in his infinite understanding of the human condition, continues to show new generations of youth the complexities of relationships. This is a repetitive theme in many of his plays, including Othello, Romeo and Juliet, and As You Like It.