What aspects of love are defined in the relationship of Helena and Demitrius, Hermia and Lysander, and Titiana and Oberon?
Interestingly, your question could be widened to cover two more pairings in this play: Theseus and Hippolyta and Bottom and Titania. It is clear that love and its different manifestations and stages are a key theme of this play. In particular, note Helena's speech in Act I Scene i, where bemoaning her state, she states: "Things base and vile, holding no quantity / Love transposes to form and dignity." What love can do to us by blinding our eyes and looking with our mind (and therefore our imagination) is therefore key to the play - as modelled perfectly in the relationship between Titania and Bottom.
At the beginning of the play, therefore, Lysander and Hermia represent love in its first, glorious and unrealistic stages. The lovers are devoted to each other, and romantically resist authority (both parental and governmental) in their pursuit of that idealistic love. They seem to have a somewhat cliched attitude to love, as Lysander "consoles" Hermia with the idea that the "course of true love did n'er run smooth."
Demetrius and Helena represent the ability of love to wound and hurt. Helena indeed "dotes in idolatry" on Demetrius, and is ready to risk her friendship with Hermia, her virtue and her personal safety in her pursuit of his affections. She disparages herself, comparing herself to "a spaniel", who fawns upon her love. Unrequited love and the foolish things we do when in that position are thus modelled.
Titania and Oberon are a couple who model a love that although may have been strong in the past is now based on the desire to cause pain on either side. Both mention their former relationships with Theseus and Hippolyta into the fray, and Oberon's jealousy of not having the Indian boy throws the natural order into despair (Act II Scene 1). Oberon's revenge seems petty and in some ways unnecessarily cruel, though it is interesting that he is the controlling agent that brings happiness to Demetrius, Helena, Lysander and Hermia, and eventually, his own relationship with Titania. The relationship between Oberon and Titania perhaps then represents an older love that has allowed itself to become spoiled by petty jealousies and resentments.
Helena and Demetrius represent the dark, abusive side of love. Demetrius has fallen in love with Hermia, and as often is the case when that happens, despises his previous love, Helena. Helena, as is also typical, won't give up, and she continues, in this case literally, running after Demetrius. She expresses love as abjection: she is willing to be beaten like a spaniel as long as Demetrius will keep her around. We could call this a dysfunctional love relationship, but in the end it all works out.
Hermia and Lysander represent love at its heedless heights of passion. They are willing to take great risks and defy authority to be together. We could call this the "head over heels" stage of love—until a love potion turns Lysander away.
Titania and Oberon have a seasoned love relationship, but not without its dark side. They have power struggles, in this case over a young Indian boy Titania has promised his dead mother to care for. Here we see love in conflict. Because this is set in a patriarchal context, Oberon wins, but Titania shows resolve and courage in standing up to her husband. In the end, love triumphs, yet Titania is forced to break a promise to her friend.