In act 2, what is similar between Lysander & Hermia, Demetrius &Helena, & Oberon & Titania?
An interesting question. The most basic similarity is that these are all pairs of lovers. Almost as basic is that there is a disruption in what should be their placid and ideal loves. They cannot be together as they should be. Of course, they are all also in the same woods, which is not a minor thing, because it sets them all up to have their problems be resolved through magic.