AKA "boobies", particularly if they're C-cup or larger. As we all know, a guy will grant 'most any ordinary request from a buxom gal while she's
nestling her ample chest-pillows against him (especially if it's direct
skin-to-skin contact), and so as long as she lovingly
clasps him in her arms and "supplies" him with her soft warm comforting ta-tas pressed firmly against his chest as "payment" for his efforts, he'll keep massaging her back.
Example:
I sometimes get totally achy arms from giving
Tiffany a three-quarter-hour-long back-rub, but how can I
stop when she is
continuously "recompensing" me for my services with her built-in back-rub reimbursement???