The rare times I do allow myself to masticate someone's buttocks I tend to go too far, and they get hurt. So even though I often want to masticate hard enough to excavate a new posterior orifice, I hold back.
as my friends know, I'm not a very assertive person and masticating buttocks doesn't come naturally to me. There were times when I should masticate buttocks, but it just didn't feel right? -scratches head- blaaaaaaaaah