The bulk of their interactions were during the Sasuke retrieval arc, so that's where a lot of the basis of their relationship is from for the most part. When they first met during the chunin exam, Shikamaru ended up defying the initial lowly expectations Temari set for him, causing him to really catch her attention. When she shows up to save his life from Tayuya later on, she boldly smiles at him after she very clearly impresses him by slaughtering Tayuya and is just genuinely happy around him. Considering her past, she doesn't open up to others so easily, but she acts so comfortable around him because of how she knows how he so passively lives life. Shikamaru smiles so softly back at her while being grateful and I can't see that as anything but him being completely heart struck. Fast forward a bit to the Kazekage rescue arc and they're reintroduced alongside each other as equals working together on organizing the chunin exam together, which implies that they're able to get along really well together.
It's a simple ship with a foundation of mutual respect that just ended up evolving into romance.
