They're odd characters - only appearing at the start and at the end of the play. And it is the gap between these two appearances that seem to make the most difference to the way that they react to each other.
When they first enter, Theseus refers to having wooed Hippolyta (to whom he is to be married in a few days) "with my sword". He has conquered her in a war, and taken her as his bride. Those first lines of the play, before Egeus enters, are oddly transparent. Does Hippolyta resent her husband? Are the two in love? Is the conversation awkward? All interpretations can be played across the text.
It's not until 4.1 that the two appear again, now together outside, and hunting, Hippolyta making her famous comment on the music of the dogs' barking: "So musical a discord, such sweet thunder" (4.1.116-17). Happening upon the lovers, Theseus then over-rules Egeus and lifts the decree which insists that Hermia married Demetrius.
What has changed in the mean time which explains this personality change? Not a great deal, unless you argue that Hippolyta/Theseus have been "translated" (a key word in the play) into Titania/Oberon - and that their suppressed frustration in the first scene becomes the barely-controlled fury of Titania and Oberon's argument. But that's a different story.
Hippolyta and Theseus close the play watching the mechanicals' performance - though, as they head off to bed at the end, I'd conclude them, in the lovers' words, "strange and admirable" characters.