In the stage version, Boq is watching Glinda and Fiyero dancing and flirting but they've only got eyes for each other and are pretty much ignoring Boq and Nessa.
After having watched the love of his life getting fondled on the dancefloor, Boq is on the verge of explaining to Nessa that the only reason he got close to her was in order to be near Glinda when he seems to have a change of heart. She accuses him of feeling sorry for her and she's dead right. He flat out lies to her that the reason he's so upset (and needing to confess) is because she's just so damn beautiful.
Their entire relationship is then based on this lie. Nessarose ultimately strips the Munchkins of their rights in an attempt to prevent Boq from leaving her.
Boq: I'm talking about my life. The little that's left of it. I'm not free to leave Munchkinland, none of us are. Ever since she took
power, she's been stripping the Munchkins of our rights... and we
didn't have that many to begin with! And do you know why?
Nessarose Reveals Herself In The Chair.
Nessarose: To keep you here, with me. But none of that matters anymore. Look.