The personalities of the Athenian lovers are different, but, as lovers, they are interchangeable. They are a manifestation of the state of mind in which one finds oneself when caught in the heat of passionate attachment. There is no mental balance, no sober reflection. The emotions are running the show and the comic results of that are what we see.
This applies to the fairies as well as all the humans, whatever their status. Titania's first long speech to Oberon describing the results on the earth plane of their personal quarrel, is a profound reflection of the universal affects of issues between the sexes.
I just finished reading that in school, haha. Do mean are the personality's of Hermia, Helena, Demetrius and Lysander similar? If so, they are very different. Hermia is fiesty and confident to try and get Lysander, Helena's desperate for love, and will do ANYTHING to be with Demetrius. As for Lysander, he's very strong willed and determined to be with Hermia, no matter what everyone else's opinion is. And Demetrius is rather cocky, for he feels he is better then Lysander, and again is determined to be with her. In the case of love they are all very determined to get who they want, and they all strive deeply for love. Keep in mind, the situation I mentioned above is before the fairies interfere with the "love juice" from the magic flowers.
As for why Shakespere wrote it like that, um, I guess he wanted to create a very dramatic play!
Sorry if this doesn't exactly andswer you're question, I'm not sure what you mean.