To test his pain tolerance, obviously. Because if Luke can't handle the heat, then Gaara wouldn't think of doing certain other things to him. Wouldn't want to make him cry, right?

...Are you feeling okay, Luke.

That boy is a monster, m-m-m-monster
(He ate my heart )
(Could I love him?)
That boy is a monster, m-m-m-monster
(He ate my heart )
(Could I love him?)
That boy is a monster, m-m-m-monster
(Could I love him?)
That boy is a monster